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