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# If F ( a + B − X ) = F ( X ) , Then Prove that ∫ B a X F ( X ) D X = ( a + B 2 ) ∫ B a F ( X ) D X - CBSE (Science) Class 12 - Mathematics

ConceptProperties of Definite Integrals

#### Question

If $f\left( a + b - x \right) = f\left( x \right)$ , then prove that

$\int_a^b xf\left( x \right)dx = \left( \frac{a + b}{2} \right) \int_a^b f\left( x \right)dx$

#### Solution

$\int_a^b xf\left( x \right)dx = \int_a^b \left( a + b - x \right)f\left( a + b - x \right)dx ................\left[ \int_a^b f\left( x \right)dx = \int_a^b f\left( a + b - x \right)dx \right]$
$\Rightarrow \int_a^b xf\left( x \right)dx = \int_a^b \left( a + b - x \right)f\left( x \right)dx .....................\left[ f\left( a + b - x \right) = f\left( x \right) \right]$
$\Rightarrow \int_a^b xf\left( x \right)dx = \int_a^b \left( a + b \right)f\left( x \right)dx - \int_a^b xf\left( x \right)dx$
$\Rightarrow 2 \int_a^b xf\left( x \right)dx = \left( a + b \right) \int_a^b f\left( x \right)dx$
$\Rightarrow \int_a^b xf\left( x \right)dx = \left( \frac{a + b}{2} \right) \int_a^b f\left( x \right)dx$

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#### Video TutorialsVIEW ALL [5]

Solution If F ( a + B − X ) = F ( X ) , Then Prove that ∫ B a X F ( X ) D X = ( a + B 2 ) ∫ B a F ( X ) D X Concept: Properties of Definite Integrals.
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