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In the calculation of the value of (x + y) (x-2y) - my (nx-y) (m.n are all constants) Calculate the value of (x + y) (x-2y) - my (nx-y) (m.n are constants). When substituting the value of X and Y into the calculation, careless Xiaoming and Xiaoliang misjudged the value of Y, but the calculation results were all equal to 25. Careful Xiaomin substituted the correct value of X and y into the calculation, and the result was exactly 25. In order to find out, she arbitrarily took 2009 as the value of Y, and you say it's strange, the result was still 25 (1) According to the above situation, try to explore the mystery; (2) Can you determine the values of M, N and X?
(x+y)(x-2y)-my(nx-y)
=x^2-xy-2y^2-mnxy+my^2
=x^2-(1+mn)xy-(2-m)y^2
The value of an algebraic expression is independent of Y
1+mn=0
2-m=0
m=2
n=-0.5
x=+-5
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