为什么1/cos^2x=tan^2x?

证明(1-2sin x cos x )/(cos^2x-sin^2x)=(1-tan x)/(1+tan x) 证明1-tan^2x/1+tan^2x=cos^2x-sin^2x 已知tan=2,求(cos x+sin x)/(cos x-sin x)+sin^2x 证(1/sin^2x)+(1/cos^2x)-(1/tan^2x)=2+tan^2x 求证 （1—tan^2X)/(1+tan^2X)=cos^2X—sin^2X tan(3x/2)-tan(x/2)-2sinx/(cosx+cos(2x))=? 证明1/sin^x+1/cos^2x-1/tanx^2x=2+tan^2 化简tan(3x/2)-tan(x/2)-sinx/(cos(3x/2)*cos(x/2)) sin(90度+2x) /1+cos (90度+2x)为什么得tan(45度+x) 求极限x趋于0时(1-cos(x/2))x / (tan x-sin x) 已知(x)=[sin(π-x)cos(2π-x)tan(-x+π)]/[cos(-π/2+x)] f(x)=1/cos²x-tan²x+根号2*sin(2x-π/4)