x+3/x+1·2x的平方+6x/x的平方+6x+9-x/x+1
=(x+3)/(x+1) · 2x(x+3)/(x+3)² - x/(x+1)
=2x/(x+1)-x/(x+1)
=x/(x+1)
=2/(2+1)
=2/3