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Prove that sin(A+B+C+)cos(B+C)-cos(A+B+C)sin(B+C)=sinA

  • trig -

    recall that
    sinPcosQ - cosPsinQ = sin(P-Q)

    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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