Integral Function

NovErdy

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Joined
Jan 21, 2019
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12
I'm sorry for the language in the picture, but it basically asks for f(18).
I don't know how to start at all
Thanks in advance
9cc008854928f8a873bb34bf82d2a4b2.jpg


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Is this the whole question? How do you do it if you don't know f(x)?

-Dan
 
Yes, that's the whole question.
That is what I was wondering too, I didn't even know how to start solving the question

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Okay, I've got a bug in my calculations but this should work. I'll give you the general lay-out and let you fill in the rest. If you have questions just let us know.

First f(x) has to be a quadratic. The integrals are just constants so you will have \(\displaystyle a = \int_{-1}^0 f(x) ~ dx\) and similar for b and c. Thus \(\displaystyle f(x) = ax^2 + bx + c + 1\). Calculate these integrals out in terms of a, b, and c.

You will now have three equations in a, b, c. Solve the system and there you go.

-Dan
 
What do you mean by this?

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\(\displaystyle a = \int_{-1}^0 f(x) ~ dx\)

\(\displaystyle b = \int_{-2}^0 f(x) ~ dx\)

\(\displaystyle c = \int_{-3}^0 f(x) ~ dx\)

And we have that \(\displaystyle f(x) = a x^2 + bx + (c + 1)\)

Plugging this into the first equation gives
\(\displaystyle a = \int_{-1}^0 (ax^2 + bx + (c + 1))~dx\)

Do the integral and you have an equation for a in terms of a, b, and c. Do all three integrals and you will have a set of 3 equations in 3 unknowns. Solve this for a, b, and c.

Give it a try and let us know if you get stuck.

-Dan
 
Sorry for the slow reply.
After integrating the equation, I get \(\displaystyle a = \frac{a}{3} - \frac{b}{2} + c + 1\).
Is it okay to subtracting a from both sides?

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Sorry for the slow reply.
After integrating the equation, I get \(\displaystyle a = \frac{a}{3} - \frac{b}{2} + c + 1\).
Is it okay to subtracting a from both sides?

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Yes of course. It's just a normal equation in 3 variables.
 
Ok, thank you for all the help! I think I've figured it out

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