How_To_Be_Good_At_Math

MEASUREMENT • USING FORMULAS TO FIND PERIMETER MEASUREMENT • USING FORMULAS TO FIND PERIMETER 167

A scalene triangle has

three sides that are all

different lengths.

We know that an equilateral

triangle has three sides that

are all the same length.

An isosceles triangle has

two sides that are equal

in length and one side that

is different.

If we call the three sides

“a”, “b”, and “c”, we can

find the perimeter by adding

the three lengths together. We

can use the formula:

Like we do with a square,

we just need to multiply the

length of one side by the number

of sides. If we call the length “a”,

the formula we can use is:

Let’s call each of the two

sides that are the same

“a”. To find the perimeter, we

multiply “a” by two then add the

length of the other side, “b”:

Equilateral triangle Isosceles triangle Scalene triangle

a a a a a

a c

b

b

Perimeter of

an equilateral

triangle

= 3a

Perimeter of

an isosceles

triangle

= 2a + b

Perimeter of

a scalene

triangle

= a + b + c

Look at this triangle. We know its

perimeter and the lengths of two sides.

Let’s find the length of the unknown side.

We can find the length of the unknown side

by simply subtracting the lengths that we

know from the perimeter: 57 – 23 – 12 = 22

So, the unknown side is 22 m long.

Using perimeter

to find a missing

measurement

If we know the

perimeter of a shape

and all of its side

lengths except one,

we can work out the

length of the missing

side with a simple

subtraction calculation.

23 m

12 m

Perimeter = 57 m

Let’s imagine the three sides

were each 4 cm long. The

perimeter would be 12 cm,

because 3 × 4 = 12

So, if the two sides that are

equal in length were 4 cm

and the different side was 3 cm,

the perimeter would be 11 cm.

So, if the triangle’s sides

were 4 cm, 5 cm, and 6 cm,

then the perimeter would be

15 cm, because 4 + 5 + 6 = 15

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166_167_Using_formulas_to_find_perimeter.indd 167 29/02/2016 18:03