How_To_Be_Good_At_Math

GEOMETRY • CALCULATING ANGLES INSIDE TRIANGLES 243

Finding a missing angle in a triangle

The rule we’ve just learned can be really useful,

because if we know the size of two of the angles

inside a triangle, we can work out the third.

TRY IT OUT

Find the

mystery angles

Now use the method

we’ve just learned to

find the missing angles

in these triangles.

We know that one

angle is 55° and

another is 75°.

What’s this missing angle? Let’s add these

two angles together:

55° + 75° = 130°

Now let’s subtract this

total from 180°:

180° − 130° = 50°

This means the

missing angle is 50°.

48º

77º

30º

90º

73º

73º

40º

102º

55º

Answers on page 320

75º

The angles inside a

triangle always add

up to 180°.

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242_243_Calculating_with_Angles_inside_triangles.indd 243 29/02/2016 18:05