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- To find the concavity of a function, I always start by evaluating its second derivative. The concavity of a function gives us valuable information about how its graph bends or curves over an interval. If the second derivative—denoted as f ” (x) —is positive over an interval, the function is concave up on that interval.
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State the first derivative test for critical points. Use concavity and inflection points to explain how the sign of the second derivative affects the shape of a function’s graph. Explain the concavity test for a function over an open interval.
Dec 21, 2020 · Notice how \(f\) is concave up whenever \(f''\) is positive, and concave down when \(f''\) is negative. Figure \(\PageIndex{8}\): A graph of \(f(x)\) and \(f''(x)\) in Example \(\PageIndex{2}\) Recall that relative maxima and minima of \(f\) are found at critical points of \(f\); that is, they are found when \(f'(x)=0\) or when \(f'\) is undefined.
If given a graph of f (x) or f' (x), determining concavity is relatively simple. Otherwise, the most reliable way to determine concavity is to use the second derivative of the function; the steps for doing so as well as an example are located at the bottom of the page.
Review your knowledge of concavity of functions and how we use differential calculus to analyze it.
- You have two functions f and g, where f''>0 and g''>0. If you take the second derivative of f+g, you get f''+g'', which is positive. So their sum i...
- In order for 𝑓(𝑥) to be concave up, in some interval, 𝑓 ''(𝑥) has to be greater than or equal to 0 (i.e. non-negative) for all 𝑥 in that inter...
- In general, no. Points where f"(x)= 0 are specifically called inflection points. But in the example provided, -2 is also a critical point (Observe...
- Product rule: d/dx[f(x)*g(x)] = f'(x)g(x) + f(x)g'(x) f(x) = (x-1)³ and g(x) = x
- Definition 1
- Definition 2
- Fact
- Second Derivative Test
To show that the graphs above do in fact have concavity claimed above here is the graph again (blown up a little to make things clearer). So, as you can see, in the two upper graphs all of the tangent lines sketched in are all below the graph of the function and these are concave up. In the lower two graphs all the tangent lines are above the graph...
Now that we have all the concavity definitions out of the way we need to bring the second derivative into the mix. We did after all start off this section saying we were going to be using the second derivative to get information about the graph. The following fact relates the second derivative of a function to its concavity. The proof of this fact ...
So, what this fact tells us is that the inflection points will be all the points where the second derivative changes sign. We saw in the previous chapter that a function may change signs if it is either zero or does not exist. Note that we were working with the first derivative in the previous section but the fact that a function possibly changing ...
The third part of the second derivative test is important to notice. If the second derivative is zero then the critical point can be anything. Below are the graphs of three functions all of which have a critical point at x=0x=0, the second derivative of all of the functions is zero at x=0x=0and yet all three possibilities are exhibited. The first i...
If f ′ (x) is positive on an interval, the graph of y = f(x) is increasing on that interval. If f ′ (x) is negative on an interval, the graph of y = f(x) is decreasing on that interval. The second derivative tells us if a function is concave up or concave down.
How do you describe the concavity of the graph and find the points of inflection (if any) for #f(x) = x^3 - 3x + 2#? How do you determine the concavity for #g(x)=-x^2+3x+4#? How do you find concavity when #f(x)= x^(7/3) + x^(4/3)#?
