Trig
posted by K.T. .
prove sin(α+β)sin(αβ)=cos^2βcos^2α

sin(a+b)sin(ab)
(aina cosb + cosa sinb)(sinacosb  cosa sinb)
sin^2acos^2b + sinacosasinbcosb  sinacosasinbcosb  cos^2asin^2b)
(1cos^2a)cos^2b  (1cos^2b)cos^2a
cos^2b  cos^acos^b  cos^2a + cos^2acos^2b
cos^2b  cos^2a
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