Initial value problems
Given a function, you can find the most
general antiderivative, but the constant is arbitrary
without more information.
Example: DO:
Find the most general antiderivative of f(x)=5x2+2x−3. (Don't look ahead until finished!)
Solution: You should have found
F(x)=53x3+x2−3x+C.
DO: Differentiate to
see if this is right. (In fact, differentiate to check
your work every time you antidifferentiate.)
Suppose now we ask you to find the antiderivative F(x) of
f(x)=5x2+2x−3 given that F(3)=30,
which we call an initial condition
or initial value.
Given F(x) that you found above and the initial condition, you
can now find the value of C and thus find the specific
antiderivative desired. To do this, plug 3 into your
general antiderivative F above, which you know is equal to 30.
F(3)=53∗33+32−3∗3+C=5∗9+9−9+C=45+C=30 <---
initial condition
so C=−15. Our specific antiderivative is
F(x)=53x3+x2−3x−15.
DO: Check to see that your
initial condition holds for F, and that you get f when you
differentiate.
Such an exercise might also be stated as:
Example: Find g(x) given that g′(x)=12x5+2x−3 and g(1)=2.
DO: Think this
different notation through carefully. Then work this
slightly different problem yourself before looking
further. Check your work by differentiating.
Solution: g(x)=2x6+x2−3x+2.
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