sin^2x+cos^2x=1为什么,怎么求?

已知tan=2,求(cos x+sin x)/(cos x-sin x)+sin^2x Sin x-sin y=2/3 cos x-cos y=1/2 求cos(x-y) 为什么y = x - sin(x/2)cos(x/2) = x - (1/2)sin(x) 其中sin(x/2)cos( lim(sin(x^2*cos(1/x)))/x怎么做? sin(x+y)sin(x-y)=k,求cos^2x-cos^2y 已知sin X+cos X=m,|m|小于等于根号2且|m|不等于1,求sin X^3+cos X^3,sin X^4+ (sin^2x+cos^2x )(sin^2x-cos^2x)怎么得sin^2x-cos^2x? Sin^x怎么变化成 1-cos(2x) /2 y =(cos^2) x - sin (3^x),求y' 求证（cos^2 x-sin^2 x)(cos^4 x+sin^4 x)+1/4 sin 2x sin 4x=cos 2 已知函数f(x)=1/2(sin x+cos x)-1/2|sin x-cos x|,求f(x)值域 求下列函数的奇偶性f(x)=(1+sin x-cos x)/(1+cos x+sin x),x属于[(-π/2),(π/