trig
posted by Mike .
Prove that sin(A+B+C+)cos(B+C)cos(A+B+C)sin(B+C)=sinA

recall that
sinPcosQ  cosPsinQ = sin(PQ)
LS = sin(A+B+C+)cos(B+C)cos(A+B+C)sin(B+C)
= sin((A+B+C) (B+C))
= sinA
= RS
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