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Calculus Level 4

If the function f ( x ) f(x) satisfies f ( 0 ) = 5 f(0) = 5 and f ( x ) = 6 x + 2 + x 2 sin 2 x f'(x) = 6x + \sqrt {2 + x^2 } \sin ^2 x , calculate: 2 2 f ( x ) d x \int\limits_{ - 2}^2 {f(x) \, dx}


The answer is 36.

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1 solution

Rimson Junio
Jul 14, 2015

Note that the derivative of an odd function is an even function from which it follows that the integral of an even function is an odd function plus some constant C. Notice that 2 + x 2 sin 2 x \sqrt{2+x^2}\sin^2 x is an even function. Thus, f ( x ) = 3 x 2 + o d d f ( x ) + C f(x)=3x^2+oddf(x)+C . Using the fact that f ( 0 ) = 5 f(0)=5 yields C = 5 C=5 . Solving now for the integral 2 2 f ( x ) d x = 2 2 ( 3 x 2 + o d d f ( x ) + 5 ) d x = 36 \int\limits_{-2}^2{f(x)\ dx}=\int\limits_{-2}^2{(3x^2+odd f(x)+5)\ dx}=36 *Note that a a o d d f ( x ) d x = 0 \int\limits_{-a}^a{oddf(x)\ dx}=0

That's a pretty great way! :)

Pulkit Gupta - 5 years, 5 months ago

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