兩式相加 500x+500y=500 x+y=1 y=1-x代入(1) ““231x+269(1-x)=193”” x=2, y=1-x=-1 (2,-1) 我不懂““231x+269(1-x)=193”” 如何變成x=2, y=1-x=-1 (2,-1)