sinx=sin(2kπ+x) ( 1)

sin(-x)=-sinx (2)

sin(π-x)=sinx (3)

cosx=sin(π/2-x) (4)

sin(x+y)=sinxcosy+sinycosx (5)

Derive cos(x-y) from the above formula.

cos(x-y)= sin(π/2-x+y)= sin(π/2-x)cosy+siny cos(π/2-x)

=cosxcosy+sinysin[π/2-(π/2-x)]

=cosxcosy+sinysinx

Don't read the formula in the book when you do the problem at ordinary times, try to deduce it yourself, and you will remember it soon, and I promise I won't forget it within four years.