用代入法解下列方程组1题2x-y=6,x+2y=-2.2题x+y=420,30%x+40%y=160*80%
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![用代入法解下列方程组1题2x-y=6,x+2y=-2.2题x+y=420,30%x+40%y=160*80%](/uploads/image/z/326856-48-6.jpg?t=%E7%94%A8%E4%BB%A3%E5%85%A5%E6%B3%95%E8%A7%A3%E4%B8%8B%E5%88%97%E6%96%B9%E7%A8%8B%E7%BB%841%E9%A2%982x-y%3D6%2Cx%2B2y%3D-2.2%E9%A2%98x%2By%3D420%2C30%EF%BC%85x%2B40%EF%BC%85y%3D160%2A80%EF%BC%85)
用代入法解下列方程组1题2x-y=6,x+2y=-2.2题x+y=420,30%x+40%y=160*80%
用代入法解下列方程组1题2x-y=6,x+2y=-2.2题x+y=420,30%x+40%y=160*80%
用代入法解下列方程组1题2x-y=6,x+2y=-2.2题x+y=420,30%x+40%y=160*80%
1:{2X-Y=6 (1) (1)变(3):2X-6=Y (3)
X+2Y=-2.2 (2) (3)代入(2)得:
X+2(2X-6)=-2.2 X+4X-12=-2.2
5X=-2.2+12 5X=9.8 X=1.96 将X=1.96代入(1)得:
2×1.96-Y=6 Y=3.92-6 Y=-2.08
X=1.96 Y=-2.08
2:{X+Y=420 (1) (1)变(3):X=420-Y (3)
30%X+40%Y=160×80% (2) 两边同乘以100得:
30X+40Y=12800 (2) 将(3)代入(2):
30(420-Y)+40Y=12800 12600-30Y+40Y=12800
10Y=200 Y=20 将Y=20代入(1):
X+20=420 X=400
X=400 Y=20
你可以代进去验算一下.