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Advanced Math / Nonlinear equations in one variable and systems of equations in two variables Difficulty: Medium

Equation 1: x plus y equals 17. Equation 2: x y equals 72.

If one solution to the system of equations above is the ordered pair x comma y , what is one possible value of x ?

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Explanation

The correct answer is either 8 or 9. The first equation can be rewritten as y equals, 17 minus x. Substituting 17 minus x for y in the second equation gives x times, open parenthesis, 17 minus x, close parenthesis, equals 72. By applying the distributive property, this can be rewritten as 17 x, minus x squared, equals 72. Subtracting 72 from both sides of the equation yields x squared, minus 17 x, plus 72, equals 0. Factoring the left-hand side of this equation yields open parenthesis, x minus 8, close parenthesis, times, open parenthesis, x minus 9, close parenthesis, equals 0. Applying the Zero Product Property, it follows that x minus 8, equals 0 and x minus 9, equals 0. Solving each equation for x yields x equals 8 and x equals 9 respectively. Note that 8 and 9 are examples of ways to enter a correct answer.