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jmc

algebra senior

Problem

For what values of is ? Express your answer in interval notation.
Solution
After moving the constant over, we get a quadratic expression and solve for the roots: The quadratic expression equals 0 at and , meaning it changes sign at each root. Now we look at the sign of the quadratic when , when , and when . When , and are both negative, so the product is positive. When , becomes positive, while remains negative - the product is negative. When , both factors are positive, so the product is positive. So, when or , which means our answer in interval notation is .

Alternatively, consider that the coefficient of is positive, so a graph of would open up. When there are two distinct roots, the shape of the parabola means that product is negative when is between the roots and positive when is less than both roots or greater than both roots.
Final answer
(-\infty, -5) \cup (7, \infty)