4a的平方+a+1/16
=(2a+1/4)²
-9b²+4a²
=(2a+3b)(2a-3b)
(x²y-xy²)/( x²-y²)
=xy(x-y)/(x+y)(x-y)
=xy/(x+y)第三个题还要代值x=（2-根号3 ）分之1xy=1 不用管y了吗x=1/(2-√3)=2+√3y=1/x=2-√3x+y=2+√3+2-√3=4 (x²y-xy²)/( x²-y²)=xy(x-y)/(x+y)(x-y)=xy/(x+y) =1/4