Question

How to calculate the sines and cosines of sums of angles?

Answer 1

$lets \: take \: a \: triangle \:$
《ABC》

A
|\
| \
| \
| \
| \
| \
|___\ B
C

RIGHT ANGLED AT C

In which angle A and angle B are always complementary..

So for sum of those angles you know it is 90

therefore. cos90=0
sin90=1