(x+y)/[1/(x^2+y^2)] /(x^4-y^4)
=(x+y)(x^2+y^2)/[(x^2-y^2)(x^2+y^2)]
=(x+y)/(x^2-y^2)
=1/(x-y)
=1/(8-11)
=-1/3
2) 先把括号打上,否则很难明白意思（x²+6x+9）分之（x²-9）×（x²3x）分之（3x³+9x²）（x²-9)/(x²+6x+9)* (3x³+9x²)/ (x²-3x)]= (x+3)(x-3)*3x^2(x+3) /[(x+3)^2*x*(x-3)]=3x=3*(-1/3)=-1