1.
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sin(x+y) = sinx.cosx + siny.cosy
sin(x-y) = sinx.cosy - siny.cosx
cos(x+y) = cosx.cosy - sinx.siny
cos(x-y) = cosx.cosy + sinx.siny
tan(x+y) = tanx + tany / 1 - tanx.tany
tan(x-y) = tanx - tany / 1 + tanx.tany
cot(x+y) = cotx.coty - 1 / cotx.coty
cot(x-y) = 1 + cotx.coty / coty - cotx
