Question

已知cos(a-b/2)=-1/9,sin(a/2-b)=2/3,0小于b小于90度小于a小于180度,求cos(a+b

Answer 1

由于a/2+B/2=(a-B/2)-(a/2-B),故

cos(a/2+B/2)=cos[(a-B/2)-(a/2-B)]

=cos(a-B/2)cos(a/2-B)+sin(a-B/2)sin(a/2-B)

由π/2