Math(Please check)

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1) Use the cofunction identites to evaluate the expression without the aid of a calculator.

cos^2 18 degrees + cos^2 72 degrees.

  • Math(Please check) -

    Error

  • Math(Please check) -

    The cofunction identities that are useful here are
    cos(90-x) = sinx and
    sin(90-x) = cosx
    and the fact that cos(A-B) = cosAcosB + sinAsinB


    so cos^2 72 = (cos72)(cos72) = (sin18)(sin18)

    then your
    cos^2 18 degrees + cos^2 72 degrees
    = cos18cos18 + sin18sin18
    = cos(18-18)
    = cos 0
    = 1

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