Question

Describe two ways to determine the maximum or minimum of the quadratic function f(x)= -3x²+27x-42. Find the maximum or minimum using each method

Answer 1

Find by using 1st derivative test (tests slope) or 2nd derivative test (test concavity). Maximum is 18.75.

Explanation:

The maximum of the equation is found when the slope is 0; therefore, take the derivative of the equation, find the critical point and find whether the function is increasing or decreasing around the critical point. To do this use test values within the interval of (-∞, 4.5) and (4.5, ∞). You then find that (-∞, 4.5) is increasing meaning there is a maximum. Plug in 4.5 into f(x) to find the y value.

Second method is using the 2nd derivative test. Find the second derivative and use the same method of testing intervals but instead plugging in test values to the 2nd derivative. You then find that f''(x) is < 0 meaning the function is concave down, thus having a maximum. Plug in 4.5 to find the y value.