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PCAT Quantitative Reasoning – Function Minima and Maxima
- by
- Apr 19, 2018
- PCAT Question of the Day
Which maximum or minimum y value does the function f(x) = 3x2 – 2x + 8 contain?
A. A minimum at y = 3
B. A minimum at y = 29
C. A maximum at y = 3
D. A maximum at y = 29
Click for Explanation
B is correct. To find extrema for a function, we can solve for the first derivative and set the first derivative equal to 0. Essentially this finds minima and maxima where the tangent line is horizontal, or has a slope of 0:
f(x) = 3x2 – 2x + 8
f’(x) = 6x – 2
0 = 6x – 2
2 = 6x
x = 3
So we know that some minimum or maximum occurs at x = 3, but we need to find the y-value:
f(3) = y = 3(3)2 – 2(3) + 8 = 27 – 6 + 8 = 29
Now, we don’t know quite yet whether this is a minimum or maximum value. To do this, we should find the second derivative. If its value is positive, this point represents a minimum, and if the value is negative, this point represents a maximum:
f’(x) = 6x – 2
f’’(x) = 6
Since the second derivative equals a positive value, the point at (3, 29) is a minimum.
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