设第一、二、三边长为A、B、C.则
A=a+2b```````````①
B=A+(b-2)````````②
C=B-5````````````③
①代入②得B=a+3b-2```````````④
④代入③得C=a+3b-7```````````⑤
周长=A+B+C=①+④+⑤=3a+8b-9