[a+2b]/[a+b]+[2b的平方]÷[a的平方-b的平方]
=(a+2b)(a-b)/(a²-b²)+[2b的平方]÷[a的平方-b的平方]
=a(a+b)/[(a+b)(a-b)]
=a/(a-b)
=-2/(-2-1/3)
=6/7