cos^4 a-sin^4 a=?sin15°-cos15°=?

sin^4+sin^2cos^2+cos^2a= 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=? （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 sin^4a-cos^4a=sin^2-cos^2a sin(a)*cos(a)=4,那么sin(a) cos(b)=? =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) 由cos(a+b)=cos a cos b-sin a sin b cos(a-b)=cos a cos b+sin a sin b解题设a为锐角,证：1、2分之根3乘cos a + 2分之1乘sin a=cos(6分之π-a)2、cos a-sin a=根号2cos(4分之π+a) 求证1/sin^2a+3/cos^2a>=4+2根号下31/sin²a +3/cos²a=(sin²a+cos²a)/sin²a +3(sin²a+cos²a)/cos²a=1+cos²a/sin²a+3+3sin²a/cos²a=4+cos²a/sin²a+3sin²a/cos²a≥4+2√[(cos cos^4A-sin^4a 化简 已知(4sin a－2cos a)／(5cos a +3sin a)=6/11,求cos＾4 a－sin＾4 a 已知sin a-2cos a=0 求4sin^2 a-3sin a x cos a-5cos^2 a的值