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Can someone walk me through how to do the following problem so I can attempt a few more practice problems?

If: $\int_{1}^{5} f(x) dx = 12 $ and $\int_{4}^{5} f( x) dx = 3.6$ find: $\int_{1}^{4} f( x) dx$

Would it simply be $12 - 3.6$ ?


EDIT

If: $\int_{0}^{9} f(x) dx = 37 $ and $\int_{0}^{9} g( x) dx = 16$ find: $\int_{0}^{9} 2f(x)+3g(x) dx$

Would this simply be: $2 \times 37 + 3 \times 16$?

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    @BrianM.Scott Thanks for the confirmation. You guys are extremely helpful here. I'm glad this resource is available, I wish I'd known about it sooner.2012-05-07

2 Answers 2

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This is done using additivity of integration on intervals i.e. if $c \in [a,b]$ and

$\displaystyle \int_a^b f(x) dx$, $\displaystyle \int_a^c f(x) dx$ and $\displaystyle \int_c^b f(x) dx$ are well- defined, then $\int_a^b f(x) dx = \int_a^c f(x) dx + \int_c^b f(x) dx$ Hence, in your case, you have that $\int_1^5 f(x) dx = \int_1^4 f(x) dx + \int_4^5 f(x) dx$ Hence, we get that $12 = \int_1^4 f(x) dx + 3.6$ i.e. $\int_1^4 f(x) dx = 8.4$


For the second problem as Brian pointed out in the comments, it follows from linearity of integration i.e. $\int \left( a f(x) + b g(x) \right)dx = a \int f(x)dx + b \int g(x) dx$ provided all the integrals are well-defined.

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This is the additive property

$\int_a^bf(x)dx+\int_b^cf(x)dx=\int_a^cf(x)dx $

The integral in the interval $[1,4]$ is the difference: integral in $[1,5]$ - integral $[4,5]$.