先化简再求值【 [x^2-9] / [ x^2+6x+9]】 ×【[3x^3+9x^2] / [x^2-3x]】

[x^2-9] / [ x^2+6x+9]】 ×【[3x^3+9x^2] / [x^2-3x]】=(x+3)(x-3)/(x+3)²*(3x²(x+3)/x(x-3))=(x-3)/(x+3)*3x(x+3)/(x-3)=3x;【 [x+y] / [ x^4+y^4]】 ÷【1 / [x^2+y^2]】=(x+y)(x²+y²)/(x^4+...