Question 1020984
{{{sin(a+b)*sin(a-b) = (sinacosb+cosasinb)(sinacosb-cosasinb) = sin^2(a)cos^2(b) - cos^2(a)sin^2(b)}}}

= {{{sin^2(a)cos^2(b) +cos^2(a)cos^2(b)- cos^2(a)sin^2(b)-cos^2(a)cos^2(b)}}}

={{{cos^2(b)(sin^2(a) +cos^2(a))- cos^2(a)(sin^2(b)+cos^2(b))}}}

={{{cos^2(b) - cos^2(a)}}}