4x-y是7的倍数,则令 4x-y=7m.于是 y=4x-7m .
2x+3y = 2x+3*(4x-7m ) = 14x-21m =7 (2x-3m) .
8x^2+10xy-3y^2 = (4x-y)*(2x+3y) = 7m*7(2x-3m) =49m(2x-3m) ,是49的倍数