求证1＋2sin a cos a／cos^2 a－sin^2 a＝1＋tan a／1－tan a

求证1＋2sin a cos a／cos^2 a－sin^2 a＝1＋tan a／1－tan a 求证：tan a/2＝(1-cos)/sin a=sin a/(cos a+1) 为什么 tan a +1/tan a=sin^2 a +cos^2 a/(sina cos a) 求证[1]1-2sin acos a/cos²a -sin²a =1-tan a/1+tan a.[ 证明tan a/2=sin a/(1+cos a) tan a/2=sin/1+cos a=1-cos a/sin a 求证：sin^2A+cos^2A+sinAcosA/sin^2A+cos^2A=tan^2A+1+tanA/tan^2A 求证:sin a(1+tan a)+cos a(1+1/tan a)=1/sin a+1/cos a 求证1/（sin^2)a+1/(cos^2)a-1/(tan^2)a=2+(tan^2)a 证明恒等式tan a*sin a/tan a-sin a=1+cos a/sin a 证明:cos^4 a-sin^4 a=cos^2 a(1-tan a)(1+ tan a) tan a/2=(1-cos a)/sin a=sin a/(1+cos a)如何证明?