Originally Posted by
Johnny Walker Black For the function f(x) find the intervals on which it is inc/dec.
f(x)= x⁴ - 4x³ + 10
f'(x) = 4x³ - 12x² or 4x²(x - 3) => x=3
so i have the intervals (-∞, 3) and (3, ∞) i'm using the test values 1 and 4 and im plugging my test values into f'(x).
I'm getting -8 for the test value 1 so its decreasing, and 64 for the test value 4 so it's increasing.
DEC on (-∞, 3) and INC on (3, ∞).
My professor got INC on (-∞ , 3) ; DEC on (3, ∞) can you spot what i'm doing wrong?