sina+cosa=1/2 则 sin^3a+cos^3a=
问题描述:
sina+cosa=1/2 则 sin^3a+cos^3a=
答
sina-cosa=1/2,
两边平方:
1-2sinacosa=1/4
sinacosa=3/8
(sina)^3-(cosa)^3
=(sina-cosa)(sin^2 a+sinacosa+cos^2 a)
=(sina-cosa)(1+sinacosa)
=(1/2)(1+3/8)
=11/16