Sin^4a+cos^4a怎么=（sin^2a+cos^2a)-2sin^2acos^2a的.求详解.在线等

sin^4+sin^2cos^2+cos^2a= =1-（sin^6a+cos^6a） ------（1）=1-（sin^a+cos^a)(sin^4 a-sin^acos6a+cos^4 a) --------（2）由（1）怎么能到（2）呢=1-（sin^6a+cos^6a）=1-（sin^2a+cos^2a)(sin^4 a-sin^2acos^2a+cos^4a) Sin^4a-cos^4a= 8sin(45+a)sin(45-a)=1,sin^4a-cos^4a= cosA+cos^2A=1,sin^2A+sin^4A+sin^8A (cos a)^4+(sin a)^2*(sin a)^2+(sin a)^2=? Sin^4a+cos^4a怎么=（sin^2a+cos^2a)-2sin^2acos^2a的.求详解.在线等 （1-sin^6a-cos^6a)/(1-sin^4a-cos^4a)=? sin^4a-cos^4a=sin^2a-cos^2a 证明sin^4a-cos^4a=sin^2a-cos^2a sin∨4a-cos∨4a=sin²a-cos²a 求证 tan a =3求4sin a-2cos a/5cos+3sin a (cos a+sin a)/(cos a-sin a) 怎么算~ 若sin^4a/sin^2b+cos^4a/cos^2b=1,证明sin^4b/sin^2a+cos^4b/cos^2a=1.(不用向量怎么证?) sin^4a-cos^4a=sin^2-cos^2a sin(a)*cos(a)=4,那么sin(a) cos(b)=? (sin a)/(1+cos a)=(1-cos a)/sin a怎么证sin a 1-cos a------ = -----------1+cos a sin a sin(a-π/4)+cos(a+π/4)怎么解?