1,
A+(-2x²+3x-4)=3x²-5+4x
A=3x²-5+4x-(-2x²+3x-4)
=3x²-5+4x+2X²-3X+4
=5X²+X-1;
2,
A-B+3C=0
C=(B-A)/3
=[5mn-6m²-9n²-(3m²-2n²+mn)]/3
=[5mn-6m²-9n²-3m²+2n²-mn]/3
=(4mn-9m²-7n²)/3
3
个位数字+十位数字=X
个位数字-十位数字=3
两式相减:2(十位数字)=X-3,十位数字=(X-3)/2;
两式相加:2(个位数字)=X+3,个位数字=(X+3)/2;
含x的代数式表示这个两位数 :
10(X-3)/2+(X+3)/2
=(10X-30+X+3)/2
=(11X-27)/2