M=X^2+XY N=Y^2-XY
题1:X^2 - 3XY + 4Y^2 = (X^2 + XY) + 4(Y^2 - XY) = M - 4N
题2:2X^2 + 4XY -2Y^2 = 2(X^2 + 2XY - Y^2)= 2[(X^2 + XY) - (Y^2 - XY)]
= 2(M - N)
= 2M -2N
M=X^2+XY N=Y^2-XY
题1:X^2 - 3XY + 4Y^2 = (X^2 + XY) + 4(Y^2 - XY) = M - 4N
题2:2X^2 + 4XY -2Y^2 = 2(X^2 + 2XY - Y^2)= 2[(X^2 + XY) - (Y^2 - XY)]
= 2(M - N)
= 2M -2N