Answer:
(-5, 4)
Explanation:
x^2 + x < 20 can be rewritten as a quadratic in standard form: x^2 + x - 20 < 0. Recognize that the graph of x^2 + x - 20 is that of a parabola that opens up. Because the y-intercept (0, -20) is below the x-axis, we know for certain tht the graph intersects the x-axis in two places. Our job is to determine the two horizontal intercepts, which in turn determine the solution set.
x^2 + x - 20 = 0 factors as follows: (x + 5)(x - 4) = 0, whose roots are -5 and 4.
These two roots form 3 intervals: (-infinity, -5), (-5, 4) and (4, infinity). We now must identify on which intervals x^2 + x - 20 is less than 0 (that is, on which intervals the graph is below the x-axis). Choose three test values for x: the first could be -6 (which is in the set (-infinity, -5) ); x^2 + x - 20 is + there, and so the graph is above the x-axis and x^2 + x - 20 is positive. Reject this.
Next, test x = 0; x^2 + x - 20 is negative there, meaning that x^2 + x - 20 < 0 and that x^2 + x < 20. Thus, the solution is (-5, 4). It can be shown that x^2 + x - 20 is positive again for x-values within the interval (4, infinity).
Evaluating