The missing side = 21°
Explanation:
The sum angles in a triangle is 180°
angles in the larger triangle:
20° + 40° + the third angle = 180°
60° + third angle = 180°
third angle = 180 - 60
third angle = 120°
When two lines cross, their opposite angles are equal. The colours in green are vertical angles. The opposite angle to the third angle we got will also be 120°
The sum of angles in the smaller triangle
120 + 39 + the missing side = 180
159 + the missing side = 180
The missing side = 180 - 159
The missing side = 21°
