25x^2-40xy+16y^2

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find 2 numbers when mutiply then together will give me (400, ie. 25*16), then when we add or subtract them they will give me -40, ie. the coefficient of xy.  think -20xy - 20xy will work. Replace the -40xy, then factorise by grouping the first 2 terms and last 2 terms.  Ans: 25x^2 - 20xy -20xy +16y^2  =(25x^2 - 20xy) - (20xy -16y^2) =5x(5x-4y) - 4y(5x-4y)  =(5x-4y)(5x-4y)