докажите тождество 
a)cos(α-β)+sin(-α)sinβ=cosαcosβ
б)sin(30°-α)-cos(60°-α)= ₋√3sinα
в)sin(α-β)-cosα sinβ (₋β)=sinα cosβ
г)sin (30°-α)+sin(30°+α)=cosα