sin(α-β)cosα-cos(α-β)sinα=3/5
sin(-β) = 3/5
-sinβ = 3/5
cos2β
= 1- 2(sinβ)^2
= 1-2(-3/5)^2
= (25-18)/25
= 7/25
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sin(α-β)cosα-cos(α-β)sinα
= sin(α-β-α) = sin(-β)