（cos36°）平方+（cos72°）平方等于多少?

原式=1-sin²36+sin²18
=1+(sin18+sin36)(sin18-sin36)
=1+[sin(27-9)+sin(27+9)][sin(27-9)-sin(27+9)]
=1+(sin27cos9-cos27sin9+sin27cos9+cos27sin9)(sin27cos9-cos27sin9-sin27cos9-cos27sin9)
=1-4sin27cos9cos27sin9
=1-(2sin27cos27)(2sin9cos9)
=1-sin18sin54
=1-2sin18cos18cos36/2cos18
=1-sin36cos36/2cos18
=1-sin72/4sin72
=1-1/4
=3/4