Sum and Difference Identity Proof Part 1

Here I motivate the need for and then prove the difference identity for the cosine function: cos(A-B) = cos(A)cos(B) + sin(A)sin(B). I use solving for an exact solution of cos(15degrees) to do this.

[kingdking]

What software do you use for the presentation? It’s nice and clear. Thank you.

[elizabethhenzell]

Is the the Cos A Sine of A in the negative quadrant for Cosine?

[elizabethhenzell]

Should the Cosine A sine B not be negative as the Cosine of A is in the negative quadrant for Cosine?

[elizabethhenzell]

can we use the distance formula if the triangles are not exactly right angled? the triangles in the Unit Circle appear not to be right angled

[mohamudosmanhamud]

Thank you Teacher Verry Useful Information ...