求证：｜x+y ｜= ｜x ｜+｜y ｜成立的必要条件是 xy≥0

x+y=xy求证x=y 若x,y>=2,求证x+y 已知y>x>e,求证：x^y>y^x X=Y 求证1=2 求证y=x*sinx 不是周期函数 已知x>0y>01/x+9/y=1,求证：x+y>=16.快 已知x,y都是正整数,求证x^3+y^3>=x^2y+xy^2 已知x,y,z∈R,求证:x^2+y^2>=xy+x+y-1 求证f（x+y）+f(x-y)=2f(x)f(y)是周期函数 求证sin²x-sin²y=sin(x+y)sin(x-y) 已知x+y+z=0求证x*x*x+y*y*y+z*z*z=3xyz x,y属于R*,且x+y=1,求证:(1)(x+1/x)(y+1/y)≥25/4 设正实数x,y满足x^3+y^3=x-y,求证：x^2+4y^2 求证tan(x-y)+tan(y-z)+tan(z-x)=tan(x-y)tan(y-a)tan(z-x) 已知函数f(x)=3x,求证:f(x)+f(y)=f(x+y) f(x+y)+f(x-y)=2f(x)cosy,求证f(x)为周期函数 已知x>y>0,求证x+1/(x-y)x>=3 x-y/x-x+y/y-(x+y)(x-y)/y² y/x=2