cos^2a-sin^2a=多少?

证明Cos^A-Sin^A=1-2Sin^A=2Cos^A-1=cos^a-sin^a 已知tan a=2,求sin a+cos a /cos a -sin a 的值 cos A/2等于多少还有sin A/2等于 化简[cos(a-π)/sin(π-a)]sin(a-π/2)cos(π/2+a)= tan a/2=sin/1+cos a=1-cos a/sin a 求证：tan a/2＝(1-cos)/sin a=sin a/(cos a+1) 已知 sin a-cos a=1/2,则 sin^3 a-cos^3 a 化简(1-sin^6 a-cos^6 a)/(cos^2 a-cos^4 a)== 证明sin(4A)sin(2A)(1-cos(2A)) cos(4A)cos(2A)(1 cos(2A))=cos(2A 若sin^4a/sin^2b+cos^4a/cos^2b=1,证明sin^4b/sin^2a+cos^4b/cos^2a sin a cos a=1／8.且π/4＜a＜π/2,则cos a-sin a的值为多少 (cos a)^4+(sin a)^2*(sin a)^2+(sin a)^2=?