The integral ∫ − 2 π 2 π 1 + e x 2 sin ( x ) x 2 cos ( x ) d x converges to the value c π a − b , where a , b and c are integers. Calculate a + b + c .
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I used the same method!
Let f ( x ) = 1 + e x 2 sin ( x ) x 2 cos ( x ) . Then, we can write f ( x ) as the sum of an even and odd function: f ( x ) = 2 f ( x ) + f ( − x ) + 2 f ( x ) − f ( − x ) . The latter term vanishes due the symmetry of the bounds. We calculate 2 f ( x ) + f ( − x ) = 2 x 2 cos ( x ) and so we can use integration by parts to obtain 4 π 2 − 8 as explained Joel's solution.
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In order to solve the problem, we will focus us on a specific theorem: Let a ∈ R and let f , g be functions such that f ( − x ) = f ( x ) and g ( − x ) = − g ( x ) for all x ∈ [ − a , a ] . Then for all b > 0 : ∫ − a a 1 + b g ( x ) f ( x ) d x = ∫ 0 a f ( x ) d x . In the description of the problem a link can be found to a proof of this theorem. However, I will get over the proof quickly. Notice that ∫ − a a 1 + b g ( x ) f ( x ) d x = ∫ − a 0 1 + b g ( x ) f ( x ) d x + ∫ 0 a 1 + b g ( x ) f ( x ) d x . ( 1 ) Using the substitution u = − x on the first integral at the right hand side gives us the information that ∫ − a 0 1 + b g ( x ) f ( x ) d x = − ∫ a 0 1 + b g ( − u ) f ( − u ) d u = ∫ 0 a 1 + b − g ( u ) f ( u ) d u = ∫ 0 a 1 + b g ( u ) f ( u ) b g ( u ) d u = ∫ 0 a 1 + b g ( x ) f ( x ) b g ( x ) d x . We can thus combine both integrals in (1), which results in ∫ − a a 1 + b g ( x ) f ( x ) d x = ∫ 0 a 1 + b g ( x ) f ( x ) + f ( x ) b g ( x ) d x = ∫ 0 a 1 + b g ( x ) f ( x ) ( 1 + b g ( x ) ) d x = ∫ 0 a f ( x ) d x . ■
Now, note that we can make use of this theorem to see that ∫ − 2 π 2 π 1 + e x 2 sin ( x ) x 2 cos ( x ) d x = ∫ 0 2 π x 2 cos ( x ) d x , as the functions x 2 cos ( x ) and x 2 sin ( x ) are even and odd, respectively. Using repeated integration by parts we can show that ∫ 0 2 π x 2 cos ( x ) d x = [ x 2 sin ( x ) + 2 x cos ( x ) − 2 sin ( x ) ] ∣ ∣ ∣ ∣ 0 2 π = 4 π 2 − 2 = 4 π 2 − 8 , from which follows that a + b + c = 2 + 8 + 4 = 1 4 .