- #1
whoareyou
- 162
- 2
I just have a question regarding the function f(x) = 9 (x - 5)^{2/3}. I did the all the stuff to find concavity and I got my answer that it's never concave up and concave down on (-∞,5)U(5,∞). But I don't know what exactly my prof is saying (ie. the initial answer to the question was wrong so I corrected it by saying: "The second derivative is -2/(x-5)^(4/3). So it is obvious that f"(5) does not exist and that at all other points the function is negative." He wrote back and said that my answer was right but for the wrong reason. He saying that "If f is concave down on (-inf, inf), then all tangents lie above the curve for all real numbers. But the tangent at x=6 intersects the curve for some x-value less than 0." I don't understand what he's trying to say, I mean isn't the answer simply just because the second derivative dne at x = 5 but negative everywhere else, you have to break up the interval for concavity at x = 5?