Middle Years

# 8.03 Finding the hypotenuse

Lesson

Pythagoras' theorem states that the square of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the other two sides. The theorem can be written algebraically:

$a^2+b^2=c^2$a2+b2=c2

where $c$c represents the length of the hypotenuse and $a$a$b$b are the two shorter sides.

We can use the formula to find the length of the hypotenuse of any right-angled triangle, as long as we know the lengths of the other two sides.

Pythagoras' theorem
 $a^2+b^2$a2+b2 $=$= $c^2$c2 shorter side lengths hypotenuse

The value $c$c is used to represent the hypotenuse which is the longest side of the triangle. The other two lengths are represented by $a$a and $b$b

#### Worked example

Find the length of the hypotenuse of a right-angled triangle whose two other sides measure $10$10 cm and $12$12 cm. Round your answer to two decimal places.

Think: Here we want to find $c$c, and are given $a$a and $b$b. We can substitute the known values for $a$a and $b$b into Pythagoras' formula $a^2+b^2=c^2$a2+b2=c2 and then solve for $c$c.

Do:

 $c^2$c2 $=$= $a^2+b^2$a2+b2 Start with the formula $c^2$c2 $=$= $10^2+12^2$102+122 Fill in the values for $a$a and $b$b $c^2$c2 $=$= $100+144$100+144 Evaluate the squares $c^2$c2 $=$= $244$244 Add the $100$100 and $144$144 together $c$c $=$= $\sqrt{244}$√244 Take the square root of both sides $c$c $=$= $15.62$15.62 cm Rounded to two decimal places

Reflect: We can see that the formula gives us a hypotenuse length of $15.62$15.62 cm. This is larger than both our shorter sides, as we should expect. If we got a number smaller than one (or both) of the short sides we know we have made a mistake in our calculation.

### Exact values and rounded values

In some cases, where the sides of the right-angled triangle form a Pythagorean triad, the exact length of the hypotenuse is an integer value. But for most cases we will end up with an irrational number, that is, a surd.

In the worked example above we were asked to round our answer to two decimal places. In most cases we will want to keep our answer as an exact value.

If we are asked to give an exact answer, or to answer as a surd, we can stop our working out when we arrive at a line of working such as $c=\sqrt{11}$c=11, as this can not be simplified without losing some accuracy when it is rounded.

#### Practice questions

##### Question 1

Consider the right-angled triangle.

1. Which of the following equations do the sides of this triangle satisfy?

$c=9^2+12^2$c=92+122

A

$12^2=9^2+c^2$122=92+c2

B

$c^2=9^2+12^2$c2=92+122

C

$9^2=c^2+12^2$92=c2+122

D

$c=9^2+12^2$c=92+122

A

$12^2=9^2+c^2$122=92+c2

B

$c^2=9^2+12^2$c2=92+122

C

$9^2=c^2+12^2$92=c2+122

D
2. Solve the equation to find the length of the hypotenuse.

Enter each line of working as an equation.

##### Question 2

Find the length of the unknown side $c$c in the triangle below.

Write each step of working as an equation.

##### Question 3

Find the length of the unknown side $c$c in the triangle below.

Write each step of working as an equation and give the answer as a surd.