3x-13 : x-2
3x-6+7 : x-2
Vì x-2 : x-2 nên 3x-6:x-2
⇒7:x-2
⇒x-2 ∈ Ư(7)={1;-1;7;-7}
⇒x ∈ {3;1;9;-5}
xy+3x-7y=21
x.(y+3)-7y=21
x.(y+3)-7y-21=0
x.(y+3)-(7y+21)=0
x.(y+3)-7.(y+3)=0
(y+3)(x-7)=0
⇒y+3=0 hoặc x-7=0
y=-3 hoặc x=7
(x-10)+(x-9)+...+(x-1)=-2015
x-10+x-9+...+x-1=-2015
x.10+(-10+-9+...+-1)=-2015
x.10+(-55)=-2015
x.10=-2015+-55=-2070
x=-2070:10=-207