sin(a+60)*sin(a-60)-sin(a)^2

sinα*sinβ=(cos(α-β)-cos(α+β))/2sin(α+60)*sin(α-60)-sin²α=(cos(α+60-α+60)-cos(α-60+α+60))/2-sin²α==(cos120-cos2α)/2-sin²α=(-sin30-(1-2sin²α))/2-sin²α=(-1/2-1+2sin²α)/2-sin²α==(-3/2+2sin²α)/2-sin²α=-3/4+sin²α-sin²α=-3/4=-0,75