The function y= (x-7)(x+5) is a quadratic function (contains powers of X^2, X). It has two roots (where y=0) as is obvious from the formula: X1=7, X2=-5. It is a parabola that opens up (because the coefficient of X^2 is positive. Thus, it has a minimum between -5 and 7 and the parabola attains negative values in this interval. Therefore the answer is -5=<X<= 7 since only in this region Y is negative or zero.

Apr 20th, 2015

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