a) sin(x+b)*sin(x-b)=
)cos(x+b) ∙cos(x-b)
c)sin(x+a) ∙cos(x-a)
d)sin a∙cos(a+b)

a) sin(x+b)*sin(x-b)=1/2(cos2b-cos2x)b)cos(x+b) ∙cos(x-b)=1/2(cos2b+cos2x)c)sin(x+a) ∙cos(x-a)=1/2(sin2x+sin2a)d)sin a∙cos(a+b)=1/2(sin(2a+b)-sinb)