Question 322690
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*[tex \LARGE \ \ \ \ \ \ \ \ \ \ mn\ -\ 4m\ -\ 5n\ +\ 20]


Factor *[tex \Large m] out of the first two terms.


*[tex \LARGE \ \ \ \ \ \ \ \ \ \ m(n\ -\ 4)\ -\ 5n\ +\ 20]


Factor *[tex \Large -5] out of the last two terms.


*[tex \LARGE \ \ \ \ \ \ \ \ \ \ m(n\ -\ 4)\ -\ 5(n\ -\ 4)]


Factor *[tex \Large n\ -\ 4] out of what remains.


*[tex \LARGE \ \ \ \ \ \ \ \ \ \ (n\ -\ 4)(m\ -\ 5)]


John
*[tex \LARGE e^{i\pi} + 1 = 0]
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