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The Baudhāyana-pythagoras Theorem: questions with answers

Class 8 Maths, Chapter 9 (NCERT). 15 questions by concept, 12 with pictures. Open “Show answer” for the answer and why.

How to teach this chapter · Common misconceptions & worked visual for this chapter

Bounding the square root of 2 by squaring

Question 1

Between which two numbers, each with three decimal places, does 2\sqrt{2} lie?

  1. (a)1.415 and 1.416
  2. (b)1.413 and 1.414
  3. (c)1.414 and 1.415
  4. (d)1.410 and 1.411
Show answer

(c) 1.414 and 1.415

Square the candidates: 1.4142=1.9993961.414^2 = 1.999396, which is less than 2, and 1.4152=2.0022251.415^2 = 2.002225, which is more than 2. So 2\sqrt{2} lies between 1.414 and 1.415. It is not exactly 1.41, so it does not sit between 1.410 and 1.411.

Doubling a square

Question 2

The solid square is the given square. The dashed outer square has double its side, and the tilted square stands on its diagonal. How does the area of the outer square compare with the area of the tilted square?

A dashed square cut into four equal squares by solid mid-lines, the lower-left one solid; a dashed diagonal in each small square forms a tilted square
  1. (a)Four times
  2. (b)Half
  3. (c)Equal
  4. (d)Double
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(d) Double

The outer square holds all eight congruent triangles and the tilted square holds four of them. So the outer square has double the area of the tilted square.

Doubling a square by rearranging four congruent triangles

Question 3

A square of side 4 units is surrounded by four pieces cut from an identical square along both its diagonals. The area of the new square was worked out as 64 square units by doubling the side to 8 units. What is the correct area of the new square?

A square with a triangle placed against each of its four sides56784 units
  1. (a)between 16 and 32 square units
  2. (b)64 square units
  3. (c)16 square units
  4. (d)32 square units
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(d) 32 square units

Doubling the side gives a square of 4 times the area, not 2 times. The middle square has area 16 square units, and the four pieces add one more square of 16, so the new square has 32 square units.

Halving the area of a square

Question 4

A square is given. Which drawing inside it gives a square of half the area?

A square
  1. (a)Join the midpoints of the four sides
  2. (b)Draw a square on the diagonal
  3. (c)Draw a square on half of one side
  4. (d)Join the centre to the four corners
Show answer

(a) Join the midpoints of the four sides

The square whose corners are the midpoints of the sides takes half of each quarter of the given square, so it has half the area. A square on half a side has only a quarter of the area, and a square on the diagonal has double.

Finding a missing leg from the hypotenuse

Question 5

A right triangle has a hypotenuse of 17 cm and one shorter side of 8 cm. What is the length of the third side?

A right triangle with hypotenuse 17 cm and one side 8 cm8 cm?17 cm
  1. (a)15 cm
  2. (b)25 cm
  3. (c)√353 cm
  4. (d)9 cm
Show answer

(a) 15 cm

The hypotenuse is the longest side, so its square is the sum of the other two squares. 17² − 8² = 289 − 64 = 225, and √225 = 15 cm.

Finding the hypotenuse from the two legs

Question 6

The two sides meeting at the right angle are 3 cm and 4 cm. What is the length of the longest side?

A right triangle with legs 3 cm and 4 cm3 cm4 cm?
  1. (a)12 cm
  2. (b)25 cm
  3. (c)5 cm
  4. (d)7 cm
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(c) 5 cm

The longest side is opposite the right angle. Its square is 3² + 4² = 9 + 16 = 25, so the side is 5 cm.

Hypotenuse of an isosceles right triangle

Question 7

The two equal sides of an isosceles right triangle are 3 units each. What is the area of the square drawn on its hypotenuse?

An isosceles right triangle with equal sides 3 and a dashed square on its hypotenuse33?
  1. (a)9 sq. units
  2. (b)6 sq. units
  3. (c)36 sq. units
  4. (d)18 sq. units
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(d) 18 sq. units

The square on the hypotenuse has twice the area of the square on an equal side. That square has area 3² = 9, so the answer is twice 9, which is 18 sq. units.

Hypotenuse of the unit isosceles right triangle

Question 8

The two sides meeting at the right angle are both 1 unit. What is the length of the hypotenuse?

An isosceles right triangle with both legs 1 unit and an unknown hypotenuse11?
  1. (a)1 unit
  2. (b)√2 units
  3. (c)4 units
  4. (d)2 units
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(b) √2 units

The square drawn on the hypotenuse has twice the area of the unit square, so its area is 2 sq. units. A square of area 2 has side √2, so the hypotenuse is √2 units.

Baudhāyana triples and their scaled versions

Question 9

The Baudhāyana triple (3, 4, 5) is scaled by 10. What is the largest term of the scaled triple?

  1. (a)50
  2. (b)15
  3. (c)no such triple
  4. (d)500
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(a) 50

Scaling multiplies every term by 10, so (3, 4, 5) becomes (30, 40, 50). It is still a triple: 900 + 1600 = 2500 = 50².

Constructing a square of triple, five-times or difference area

Question 10

A square of side 4 units has a diagonal of 32\sqrt{32} units. A right triangle has this side and this diagonal as its two perpendicular sides. What is the side of the square on its hypotenuse?

A square with its diagonal, and a right triangle whose two shorter sides are the side and the diagonal of that square4√324√32?
  1. (a)48\sqrt{48} units
  2. (b)7 units
  3. (c)12 units
  4. (d)48 units
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(a) 48\sqrt{48} units

The square on the hypotenuse has area 16 + 32 = 48 sq units, three times the area 16. Its side is 48\sqrt{48} units, which is not a whole number.

Fermat's Last Theorem

Question 11

For exactly one of these equations there are infinitely many solutions in natural numbers x,y,zx, y, z. Which equation is it?

  1. (a)x2+y2=z2x^2 + y^2 = z^2
  2. (b)x4+y4=z4x^4 + y^4 = z^4
  3. (c)x3+y3=z3x^3 + y^3 = z^3
  4. (d)x5+y5=z5x^5 + y^5 = z^5
Show answer

(a) x2+y2=z2x^2 + y^2 = z^2

Every Baudhāyana triple, such as (3, 4, 5) and its multiples, solves x2+y2=z2x^2 + y^2 = z^2, so there are infinitely many. For every power above 2, Fermat's Last Theorem says there is none.

Combining two different squares

Question 12

Squares of sides 6 units and 8 units are combined by Baudhāyana's method into one square. What is the side of the new square?

A small square, a plus sign, a larger square, an arrow, and a question mark6+8?
  1. (a)14 units
  2. (b)48 units
  3. (c)10 units
  4. (d)100 units
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(c) 10 units

The new square's area is 36 + 64 = 100 square units, so its side is √100 = 10 units. The side is the hypotenuse of the right triangle with perpendicular sides 6 and 8.

Squares with corners on a dot grid

Question 13

The dots are 1 unit apart and the grid goes on without end. Which whole numbers can be the area of a square with its corners on the dots?

A 5 by 5 grid of dots joined by thin grid lines
  1. (a)even numbers only
  2. (b)all whole numbers
  3. (c)square numbers only
  4. (d)sums of two square numbers
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(d) sums of two square numbers

A side going aa across and bb up gives area a2+b2a^2 + b^2, and one of aa, bb may be 0. So the areas are 1, 2, 4, 5, 8, 9, 10, 13 and so on, and areas such as 3, 6, 7 and 11 never occur.

The Baudhayana-Pythagoras theorem from squares on the sides

Question 14

Squares are drawn on the sides of a right triangle whose shorter sides are 5 and 12. The square on the hypotenuse was given the side 17. What is the correct side of that square?

Squares on the sides of a right triangle with shorter sides 5 and 12; the hypotenuse square is marked 17 with a question mark51217?
  1. (a)17
  2. (b)13
  3. (c)169
  4. (d)119\sqrt{119}
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(b) 13

The side 17 comes from adding 5 and 12. The theorem adds the squares: 25 + 144 = 169, so the square on the hypotenuse has side 169=13\sqrt{169} = 13.

The lotus problem: depth of the lake

Question 15

The tip of a lotus stem is 1 unit above the water. Swayed by a breeze, the tip touches the water 3 units from where it stood. The depth is x, so the whole stem is x + 1. What is the value of (x+1)2−x2(x + 1)^2 - x^2?

A pond in cross-section with a lotus stem standing straight up and the same stem swayed over to touch the water1x3x + 1water
  1. (a)3
  2. (b)10
  3. (c)9
  4. (d)1
Show answer

(c) 9

The swayed stem is the hypotenuse, so 32+x2=(x+1)23^2 + x^2 = (x + 1)^2. Moving x2x^2 across, (x+1)2−x2=32=9(x + 1)^2 - x^2 = 3^2 = 9, which is also 2x+12x + 1.

Common misconception in The Baudhāyana-pythagoras Theorem

Open full misconception guide →

Can the theorem be used on any triangle?

Wrong

For two sides 3 cm and 4 cm, the third side must be 5 cm.

Why it feels right. I recognise the 3–4–5 pattern without looking at the angle.

Right

The square relation holds for the sides of a right triangle.

Worked example

The 3 and 4 sticks can open at different angles. Their far endpoints move apart as the angle opens.
  1. Hinge sticks of lengths 3 and 4.
  2. At 90° their far ends are 5 apart.
  3. Open the angle wider and the gap increases.