sin^4a+cos^4a=1-2sin^2a*cos^2a
都是数字的次方,a是未知数。
(sin^2 a+cos^2 a)^2=1
展开 就是sin^4a+cos^4a+2sin^2a*cos^2a=1
就是sin^4a+cos^4a=1-2sin^2a*cos^2a