# Thread: Integrating the second derivative

1. ## Integrating the second derivative

given f''(x) = -25sin(5x) and f'(0) = 3 and f(0) = -2, find f(pi/6)

I am trying to figure out how to integrate the second derivative into the first derivative and make it 3, then integrate into a function of 0 that equals -2.

How do I integrate the second derivative into the first derivative?

2. Originally Posted by derekjonathon
given f''(x) = -25sin(5x) and f'(0) = 3 and f(0) = -2, find f(pi/6)

I am trying to figure out how to integrate the second derivative into the first derivative and make it 3, then integrate into a function of 0 that equals -2.

How do I integrate the second derivative into the first derivative?
$\displaystyle f''(x) = -25\sin{(5x)}$

$\displaystyle f'(x) = \int{-25\sin{(5x)}\,dx}$

$\displaystyle f'(x) = 5\cos{(5x)} + C$.

You know that $\displaystyle f'(0) = 3$, so

$\displaystyle 3 = 5\cos{(5\cdot 0)} + C$

$\displaystyle 3 = 5 + C$

$\displaystyle C = -2$.

So $\displaystyle f'(x) = 5\cos{(5x)} - 2$

$\displaystyle f(x) = \int{5\cos{(5x)} - 2\,dx}$

$\displaystyle f(x) = \sin{(5x)} - 2x + C$.

You know that $\displaystyle f(0) = -2$, so

$\displaystyle -2 = \sin{(5\cdot 0)} - 2\cdot 0 + C$

$\displaystyle C = -2$.

Therefore

$\displaystyle f(x) = \sin{(5x)} - 2x - 2$.

Now find $\displaystyle f\left(\frac{\pi}{6}\right)$.

3. ## Thanks!

It seems easy when you explain it that way!

4. "All truths are easy to understand once they are discovered; the point is to discover them." - Galileo

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# integral of second derivative

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