①式+②式4a-b+2a+b=50+306a=80a=40/3代入①式4x40/3-b=50b=160/3-50b=10/3
4a-b＝502a+b＝30两个式子相加6a＝85 a＝85/6b＝30-2a＝30-2*85/6＝30-85/3＝90/3-85/3＝5/3
