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