1）(ab-b^2)÷[(a^2-b^2)/(a^2+b^2)]*(1/b)
=b(a-b)*[(a^2+b^2)/(a+b)(a-b)]*1/b
=(a^2+b^2)/(a+b)
2)x^2+3x+2分之x+1 + x^2+7x+12分之x+3
=(x+1)/[(x+1)(x+2)]+(x+3)/[(x+3)(x+4)]
=1/(x+2)+1/(x+4)
=[(x+4)+(x+2)]/[(x+2)(x+4)]
=(2x+6)/(x+2)(x+4)