cos2x=cos^2x－sin^2x吗?

求证 cos^8(x)-sin^8(x)=cos2x【1-1/2sin^2（2x)】 化简：2cos2x+2sin^2 x+cos(-x)分之sin2x+sin(π-x)=___________ [sin^2(x)-cos^2(x)] 为什么等于-cos2x f(x)=(1+cos2x)/[4sin(pai/2+x)]-asin(x/2)cos(pai-x/ 已知函数f(x)=(6cos^4x+5sin^2x-4)/cos2x 函数f(x)=cos2x+cos(x+π/3)+sin(x+π/6)+3sin^2x的最小值 求证:sin(π/4+x)/sin(π/4-x)+cos(π/4+x)/(π/4-x)=2/cos2x 已知sin(x/2)+cos(x/2)=1/2,则cos2x=? 3sin^2 x+cos^2 x=2-cos2x怎么来的三角函数公式过程 cos^4(x)-sin^4(x)等于?是cos^2(x)还是cos2x 求证 sinx(cos^2 2x-sin^2 2x) + 2cosx cos2x sin2x= sin 5x 求证:cosx+cos2x+...+cosnx={[cos(n+1)x/2]*[sin(n/2)x]}/[sin(x/2