Derive Sum of Two Angles Identities
To derive the sum of two angles identities the length of and the length of can be calculated using the properties of a right triangle with a hypotenuse of length 1. Two right triangles can be chosen for and so that the lengths can be expressed in terms of their sides.
Steps

Start by placing two righttriangles of angles and next to eachother so that their angles sum together.
There are a couple of choices of how to relate these two righttriangles together. Note in this choice, the right triangle defined by is scaled by . Ordinary with a hypotenuse of length the length of the adjacent side would be and the length of the opposite side would be , but since the hypotenuse is , they are both scaled by the value of .

Then, we can redraw the triangle defined by the angle and label the sides for reference and so they are out of the way.
Observe that if we find the two unknown lengths denoted by question marks in the image below, we have all the information needed to express the length of and .
These two lengths are given by the equations:

Observe that the righttriangle whose lengths we are interested in also has an angle of . Then, we can find the length of each of its sides since we know the length of its hypotenuse which is .
This gives us the figure below with all the lengths labeled:
Finally, we can substitue these found lengths into the equations from step 2 to get the two identities:
The sum of two angles identities express the cosine and sine of the sum of two angles in terms of their individual cosine and sine components.