sin a+cos a=(1/5) 角a是第几象限的角如题.

求证1+sin a+cos a+2sin acos a/1+sin a+cos a=sin a+cos a 已知sin a+cos a= -1/5 求1）sin acos a 2）sin a-cos a 求证:sin 2a/1+sin a+cos a=sin a+cos a-1急 sin(a-b)*cos a-cos(a-b)*sin a=1/5,则cos 2b的值是 证明Cos^A-Sin^A=1-2Sin^A=2Cos^A-1=cos^a-sin^a 利用三角函数定义证明cos a-sin a+1/(cos a+sin a+1)=1-sin a/cos a 求证:sin a (1+cos2a)=sin a cos a (sin a)/(1+cos a)=(1-cos a)/sin a怎么证sin a 1-cos a------ = -----------1+cos a sin a 求证 1+sin a+cos a+2sin a*cos a/1 +sin a +cos a = sin a+ cos a 有什么思路! 证明恒等式tan a*sin a/tan a-sin a=1+cos a/sin a 已知tan a=-1/3,求4sin a-2cos a/5cos a+3sin a= 已知tan a =-4,求下列各式的值 (1)sin^2 a (2)3sin acos a (3)cos^2 a-sin^2 a (4)4sin a已知tan a =-4,求下列各式的值 (1)sin^2 a (2)3sin acos a (3)cos^2 a-sin^2 a (4) (4sin a-2cos a)/(5cos a+3sin a) a为任意角 已知tan a =-4,求下列各式的值 (1)sin^2 a (2)3sin acos a (3)cos^2 a-sin^2 a (4)4sin a已知tan a =-4,求下列各式的值 (1)sin^2 a (2)3sin acos a (3)cos^2 a-sin^2 a (4) (4sin a-2cos a)/(5cos a+3sin a) a为任意角 sin 2a+2sin方a/ 1+tan a =2sin a cos a 化简tan a(cos a-sin a)+sin a(sin a+tan a)/1+cos a 已知sin a+cos a=1/5(0 Cos(a+b)*cos(a-b)=1/5 求cos ^2-sin^2 sin a-cos a=1/5 sin^2 a+cos^2 a=1 可以的话给先过程