原式=x/x+3十13（x+3）/（x+3）⺄^2=x/x+3十13/x+3=x+13/x+3
=x/(x+3)+13(x+3)/(x+3)²=x/(x+3)+13/(x+3)=(x+13)/(x+3)
x/(x+3)+13(x+3)/(x+3)^2=13x/(x+3)^2
好
