Construction of Perpendicular Lines: Formula, Method, Sample Questions

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Jasmine Grover

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A perpendicular line intersects another line and equalises the angles between them. Remember that there are no measurements in pure geometry, such as degrees. As a result, while it's easy to conceive of a perpendicular line as one that forms two 90-degree angles, we should resist and refer to them as two right angles instead. There are several methods for creating a line that is perpendicular to another. In general, we can create a line that intersects another line at a right angle. This line can alternatively be constructed to pass through a given location rather than along the given line. We can also build a perpendicular line to intersect the line at a specific place.

Key Terms: Perpendicular, Lines, Perpendicular Bisector, Arc, Right Angle, Angles


What are Perpendicular Lines?

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Perpendicular lines can be defined as any two lines that intersect each other in such a way that the angle formed between them is a right angle. To determine whether two lines intersecting each other at a certain point are perpendicular or not, we multiply the slopes of the lines. If the answer is equal to -1, then the lines are said to be perpendicular. The key difference between parallel and perpendicular lines is that parallel lines never intersect each other whereas perpendicular lines intersect each other at right angles.

Parallel Lines and Perpendicular lines

Parallel Lines and Perpendicular lines


Constructing a Perpendicular From a Point Given on the Line 

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Consider the line shown below . 

Perpendicular line

Perpendicular line

Given: Point P on a given line

Steps to construct a line through P perpendicular to a given line:

  • Swing an arc of any size below the line that crosses the line twice with your compass point on P. You'll create a semicircle. (Note: While drawing this arc above or below the line, drawing it below the line prevents the construction lines from intersecting.)
  • Extend the compass even more. 

arc

Arc

  • Make a little arc above the line with the compass point where the arc crossed the line on one side (the arc could be below the line if you prefer).
  • Place the compass point where the previous arc crossed the line on the other side and construct another arc without adjusting the span on the compass. Your two little arcs should cross each other.

Arc crossed

Arc crossed

  • Connect the junction of the two tiny arcs to point P with a straightedge.

Proof of Construction

This construction is essentially a ‘Bisect An Angle’ construction variation. The straight angle P was cut in half by this construction. This structure produced two 90o angles because a straight angle contained 180o. A perpendicular was generated because two right angles were formed.

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Constructing a Perpendicular From a Point Away From the Line

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Consider the line shown below.

Perpendicular From a Point Away From the Line

Perpendicular From a Point Away From the Line

Given: Point P away from the given line.

Steps to construct a line through P perpendicular to the given line:

  • Set your compass to P and swing an arc of any size across the line twice.
  • Make a little arc below the line with the compass point on one of the two points where the arc crossed the line (on the side where P is not located).

Steps to construct a line through P perpendicular to the given line

  • Place the compass point on the other side of the line where the first arc crossed the line and make another small arc below the line without changing the span on the compass. The two small arcs should cross each other (on the side of the line opposite of point P).
  • Connect the junction of the two tiny arcs to point P with a straightedge.

Straightedge

Straightedge

Proof of Construction

Mentioned below is the proof of construction:

  1. The points of intersection with the first arc are A and B, the intersection of the two smaller arcs is C, and the intersection of the perpendicular with the supplied line is D. 
  2. Because they were built as radii of the same circle, PA = PB and AC = BC. 
  3. These parts are also in line with one another. PBC is congruent to PAC by SSS when using pcas as a common side. 
  4. APC and BPC are congruent according to CPCTC. SAS now considers APD to be consistent with BPD, with a common side PD. 
  5. PDA and PDB are consistent according to CPCTC. 
  6. These two angles are supplementary because they form a linear pair. 
  7. Right angles are formed when two angles are both congruent and supplementary. 

Things to Remember 

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  • Perpendicular lines are formed when two lines connect at right angles to form right angles. Perpendicular lines are coplanar and intersect at right angles since they are in the same plane.
  • The word intersect refers to a point where two lines overlap or meet. A perpendicular is a line that intersects another at a 90° angle.
  • Arcs can be used to cut a line segment in half.
  • A ruler may be used to draw straight lines, a protractor can be used to measure and draw angles, and a compass can be used to draw arcs that are a particular distance from a point.
  • We frequently need to bisect lines or angles when constructing or drawing geometric figures.
  • To bisect anything implies to divide it into two equal sections. There are several methods for bisecting a line segment.

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Sample Questions 

Ques. The two lines that connect at right angles inside the kite are perpendicular. Make a list of how many right angles are formed at the intersection. (2 marks)
The two lines that connect at right angles inside the kite are perpendicular. Make a list of how many right angles are formed at the intersection

Ans. A kite is a symmetrical shape. As a result, it has two equal and opposite angles. Its diagonals aren't all the same length, but the larger one divides the shorter one in half at right angles. There are four right angles at the intersection point because the two diagonals of the kite AD and BC connect at right angles or 90 degrees.

Ques. Make a line AB and place a point P on the outside of it. Draw a line CD that runs parallel to AB and passes through point P. (3 marks)

Ans. Construction steps are as follows:

Construction steps are as follows

  1. Draw an AB line.
  2. Join PQ with a point Q on AB and a point P outside AB.
  3. Draw on the arc to cut AB at X and PQ at Z with Q as the centre and any radius.
  4. Draw an arc cutting QP at Y with P as the centre and the same radius.
  5. Draw an arc to cut the preceding arc at E, with Y as the centre and the radius equal to XZ.
  6. To get the desired line, join PE and produce it on both sides.

Ques. What is a perpendicular bisector? (2 marks)

Ans. A perpendicular bisector is a line segment that meets another line segment perpendicularly and divides it into two equal-sized halves. A rule, a compass, and a pencil can be used to draw a perpendicular bisector. When two lines intersect at 90 degrees or at right angles, they are perpendicular to each other. On the other hand, a bisector is a line that splits a line into two halves. A perpendicular bisector of a line segment XY intersects it at 90 degrees and divides it into two halves.

Ques. What characteristics does a perpendicular bisector have? (4 marks)

Ans. Properties of a perpendicular bisector are:

  1. Creates two congruent segments from a line segment or a line.
  2. Divides a triangle's sides into congruent sections.
  3. They form a 90-degree angle with the line being bisected.
  4. They cross the line segment exactly in the middle.
  5. The circumcenter of a triangle is the place where the perpendicular bisectors intersect.
  6. They meet inside an acute triangle, outside the triangle in an obtuse triangle, and at the hypotenuse in right triangles.
  7. The perpendicular bisector's points are equidistant from both ends of the bisected segment.
  8. For a given line segment, there can only be one.

Ques. How to construct a perpendicular bisector? (2 marks)

Ans. A ruler and a compass can be used to make a perpendicular bisector on a line segment. The built perpendicular bisector separates a line segment into two equal pieces halfway, resulting in two congruent line segments.

Ques. What is a triangle's perpendicular bisector? (2 marks)

Ans. A perpendicular bisector of a triangle is a line segment that bisects its sides while remaining perpendicular to them. They do not have to travel through the triangle's vertices, but they must pass through the midway of the sides. The perpendicular bisector of the triangle's sides is perpendicular to the middle of the triangle's sides.

Ques. How can you tell if two lines are perpendicular? Are rhombus' diagonals perpendicular to each other? Is it possible to say the same thing about a rectangle? (2 marks)

Ans. When two lines connect at a 90° angle, they are perpendicular. Because the diagonals of a rhombus intersect at a 90° angle, we can say that the diagonals of a rhombus are perpendicular to one another. Because the diagonals of a rectangle do not intersect at a 90° angle, we can say that the diagonals of a rectangle are not perpendicular to one another.

Ques. Construct the perpendicular bisector of a line segment 10 cm. (3 marks)

Ans. To create the perpendicular bisector of a line segment AB = 10 cm. Construction steps are as follows:

  1. Construct two arcs on both sides of the line segment AB and label them P and M, using A and B as centres and a radius greater than (1/2)AB.
  2. Join P and M. 
  3. Allow line PM to cut the AB portion of the line at Q.
  4. The requisite perpendicular bisector AB is then line PMQ.

Ques. Differentiate between parallel and perpendicular lines. (3 marks)

Ans. Lines that do not intersect and are always the same distance apart are called parallel lines. The symbol || is used to represent two parallel lines. Here are a few examples:

  • The straight ladder's steps
  • A rectangle's two opposite sides

Perpendicular lines are those that make a straight angle when they intersect. The symbol ⊥ is used to represent two perpendicular lines. Here are a few examples:

  • The corner of two walls
  • The letter "L"

Ques. Which of the following shapes has perpendicular sides and why? (5 marks)
(1) Square
(2) Trapezium
(3) Rectangle
(4) Rhombus
(5) Right triangle
(6) Equilateral triangle

Ans. 1. Four lines make up the square. A square has four right angles since each line is perpendicular to two other lines. The line AB is parallel to the line BC. BC, like CD, AB, and so on, is perpendicular to each other. To clarify, a square has right angles, and a square formed by drawing two more lines in the corner of a right angle is also used to represent a right angle.

Square

  1. A trapezium is a quadrangle in which one pair of opposite sides is parallel to one other while the other pair is not. The bottom of a trapezium is normally drawn using one of the parallel edges. The bases are the parallel edges, and the sides are the non-parallel edges. So, there are no perpendicular lines. Special trapeziums come in a variety of shapes and sizes. One side of a "right trapezium" is perpendicular to the bases, resulting in two right angles. So, the lines are perpendicular.

Trapezium

  1. Right angles can also be found in rectangles. A rectangle, like a square, has four right angles. This means it has lines that are perpendicular to each other.

Rectangle

  1. Rhombus has equal lines but no perpendicular lines as clearly shown in the figure. The rhombus has diagonals which bisect each other at right angles, so the diagonals are perpendicular to each other, whereas the sides are not.

Rhombus

  1. A right triangle has perpendicular lines as shown in the figure.

Right triangle

  1. An equilateral triangle has all angles equal to 600, so it does not have perpendicular lines.

Equilateral triangle

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CBSE X Related Questions

1.
The angle of elevation of the top of a building from the foot of the tower is 30° and the angle of elevation of the top of the tower from the foot of the building is 60°. If the tower is 50 m high, find the height of the building.

      2.

      The lengths of 40 leaves of a plant are measured correct to the nearest millimetre, and the data obtained is represented in the following table :

      Length (in mm)

      Number of leaves

      118 - 126

      3

      127 - 135 

      5

      136 - 144

      9

      145 - 153

      12

      154 - 162

      5

      163 - 171

      4

      172 - 180

      2

      Find the median length of the leaves. 
      (Hint : The data needs to be converted to continuous classes for finding the median, since the formula assumes continuous classes. The classes then change to 117.5 - 126.5, 126.5 - 135.5, . . ., 171.5 - 180.5.)

          3.

          A vertical pole of length 6 m casts a shadow 4 m long on the ground and at the same time a tower casts a shadow 28 m long. Find the height of the tower.

              4.
              A vessel is in the form of an inverted cone. Its height is 8 cm and the radius of its top, which is open, is 5 cm. It is filled with water up to the brim. When lead shots, each of which is a sphere of radius 0.5 cm are dropped into the vessel, one-fourth of the water flows out. Find the number of lead shots dropped in the vessel.

                  5.

                  Form the pair of linear equations for the following problems and find their solution by substitution method.

                  (i) The difference between two numbers is 26 and one number is three times the other. Find them.

                  (ii) The larger of two supplementary angles exceeds the smaller by 18 degrees. Find them.

                  (iii) The coach of a cricket team buys 7 bats and 6 balls for Rs 3800. Later, she buys 3 bats and 5 balls for Rs 1750. Find the cost of each bat and each ball.

                  (iv) The taxi charges in a city consist of a fixed charge together with the charge for the distance covered. For a distance of 10 km, the charge paid is Rs 105 and for a journey of 15 km, the charge paid is Rs 155. What are the fixed charges and the charge per km? How much does a person have to pay for travelling a distance of 25 km.

                  (v) A fraction becomes\(\frac{ 9}{11}\), if 2 is added to both the numerator and the denominator. If, 3 is added to both the numerator and the denominator it becomes \(\frac{5}{6}\). Find the fraction.

                  (vi) Five years hence, the age of Jacob will be three times that of his son. Five years ago, Jacob’s age was seven times that of his son. What are their present ages?

                      6.
                      An umbrella has 8 ribs which are equally spaced (see Fig. 11.10). Assuming umbrella to be a flat circle of radius 45 cm, find the area between the two consecutive ribs of the umbrella.
                      An umbrella has 8 ribs which are equally spaced

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