Multiply the left side out:



Rearrange like terms together:



Combine like terms and replace sin2(A)+cos2(A) by 1:





Factor out 2:



Move the cos(A) to the end of the parentheses:



Out of the first two term in parentheses,
factor out sin(A), and
out of the last two terms in parentheses,
factor out 1:



Factor out out common factor 



Reverse terms in first parentheses:



Edwin