4s+3t=5①2s-t=-5②先用②×2得：4s-2t=-10③③-①得：（4s-2t）-(4s+3t) =-10-5-2t-3t=-15-5t=-15t=3把t=3带入②得：2s-3=-5s=-1所以,s=-1,t=35x-6y=9 ①7x-4y=-5 ②①×7得：35x-42y=63 ③②×5得：35x-20y=-25④...