x分之(x²-y²)÷[x- x分之(2xy-y²）]
=x分之(x²-y²)÷[ x分之(x²-2xy+y²）]
=x分之(x+y)(x-y) * [(x-y)²分之x]
=(x-y)分之(x+y)
=(2+1)分之(2-1)
=3分之1