8.04 Finding a short side

Lesson

Worksheet

Practice

Lesson

We have been using Pythagoras' theorem to relate the three sides of a right-angled triangle together:

Up until now we have been using this formula to find the length of the hypotenuse, knowing the length of the two short sides.

We can use the same formula to find the length of a short side, knowing the length of the hypotenuse and the length of the other short side. The only difference when finding a short side is that we can put the numbers in the wrong way around if we aren't careful.

Worked example

A right-angled triangle has a hypotenuse of length $20$20 m and one short side that has a length of $11$11 m. Find the exact length of the other short side, and find its length rounded to two decimal places.

Think: Since we want to find the length of a short side, we will be solving for either $a$a or $b$b- let's choose $a$a. Since we want to find $a$a, our given values will be $b$b and $c$c which we can substitute into Pythagoras' formula $a^2+b^2=c^2$a2+b2=c2 and then solve for $a$a.

Do: We will substitute $b=11$b=11 and $c=20$c=20 into the formula for Pythagoras' theorem:

$a^2+b^2$`a`2+`b`2	$=$=	$c^2$`c`2	Start with the formula
$a^2+11^2$`a`2+112	$=$=	$20^2$202	Fill in the values for $b$`b` and $c$`c`
$a^2$`a`2	$=$=	$20^2-11^2$202−112	Subtract $11^2$112 from both sides to make $a^2$`a`2 the subject
$a^2$`a`2	$=$=	$400-121$400−121	Evaluate the squares
$a^2$`a`2	$=$=	$279$279	Subtract $121$121 from $400$400
$a$`a`	$=$=	$\sqrt{279}$√279 m	Take the square root of both sides
$a$`a`	$=$=	$16.70$16.70 m	Rounded to two decimal places

The exact length is $\sqrt{279}$√279 m, and the rounded length is $16.70$16.70 m.

Reflect: When finding a short side, our answer should always be shorter than the hypotenuse. If our answer is longer, we know we have made a mistake.

Careful!

The most important thing to remember when finding a short side is that the two lengths need to go into different parts of the formula.

If you get the lengths around the wrong way, you will probably end up with the square root of a negative number (and a calculator error).

Rearranging to find a short side

We can also rearrange the equation before we perform the substitution, to find formulas for each side length.

Rearranging Pythagoras' theorem

To find a shorter side:

$a^2=c^2-b^2$a2=c2−b2 or $b^2=c^2-a^2$b2=c2−a2

We can take the square root of both sides to give us the following formulas:

$a=\sqrt{c^2-b^2}$a=√c2−b2 or $b=\sqrt{c^2-a^2}$b=√c2−a2

Practice questions

Question 1

Consider the right-angled triangle.

Which of the following equations do the sides of this triangle satisfy?
$13^2=k^2-5^2$132=k2−52
A
$k=13^2-5^2$k=132−52
B
$k^2=13^2+5^2$k2=132+52
C
$k^2=13^2-5^2$k2=132−52
D
Solve the equation to find the length of the unknown side.

Enter each line of working as an equation.

Question 2

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

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

Question 3

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

Write each step of working as an equation and give the answer to two decimal places.