How_To_Be_Good_At_Math

inside triangles Calculating angles

TRY IT OUT

Work out the angles

If you know what type of

triangle you’re looking at, you

can sometimes work out all its

angles, even if you only know

one of them. See if you can

work out the two missing

angles here. The steps will

help you if you get stuck.

This is an isosceles triangle, so

we know that a and b are equal.

We know that a + b + c = 180°.

Angle c is 40°, so if we take 40°

away from 180°, the answer will be

the same as a + b.

Now, if we divide that

answer by two, we find

the size of angles a and b.

Answer on page 320

The third angle

can be acute

(less than 90°)

Arcs mark the

two equal

angles

The third angle can

be a right angle

Obtuse

angle

Acute

angle

Scalene

triangles can

be right-angled

40°

Scalene triangle

A scalene triangle has

no equal sides, and all its

angles are different. It can

contain one right angle, or

it can be made up of a

combination of acute and

obtuse angles.

Isosceles triangle

An isosceles

triangle has two angles

of equal size and two

sides of the same

length. The third angle

can be any size.

c

a b

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