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By splitting the Middle Term, 5X2 + 25X + 4X +20 5X(X+5) + 4(X+5) (5X+4)(X+5)
5x2=10...take 5 two times..5+5=10
To factorise x3 + 5x2 - 16x - 80 I note that -80 = 5 x -16 and 5 & -16 are the coefficients of x2 and x. Thus I have: x3 + 5x2 - 16x - 80 = (x + 5)(x2 - 16) and the second term is a difference of 2 squares, meaning I have: x3 + 5x2 - 16x - 80 = (x + 5)(x + 4)(x - 4)
5x2 = 3 then x2 = 3/5 so that x = sqrt(3/5) = ± 0.7746
5x2 + 20 = 5*(x2 + 4) which cannot be factorised further.