PV ∫ 0 ∞ 1 − x 3 d x = c a b π
The equation above holds true for positive integers a , b , and c with g cd ( a , c ) = 1 and b being square-free. Find a + b + c .
Note:
PV denotes the Cauchy principal value.
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I = ∫ 0 ∞ 1 − x 3 d x = ∫ 0 1 1 − x 3 d x + J ∫ 1 ∞ 1 − x 3 d x
⟹ J = ∫ 0 1 1 − x 3 − x d x ⟹ I = ∫ 0 1 1 − x 3 1 − x d x = ∫ 0 1 ( x + 2 1 ) 2 + 4 3 d x ⟹ I = 3 2 arctan ( 3 3 ) − 3 2 arctan ( 3 1 ) ⟹ I = 9 3 π ⟹ a + b + c = 1 3
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I = ∫ 0 ∞ 1 − x 3 d x = 2 1 ∫ 0 ∞ ( 1 − x 3 1 + x 2 ( 1 − x 3 1 ) 1 ) d x = 2 1 ∫ 0 ∞ ( 1 − x 3 1 + x 3 − 1 x ) d x = 2 1 ∫ 0 ∞ 1 − x 3 1 − x d x = 2 1 ∫ 0 ∞ x 2 + x + 1 1 d x = 2 1 ∫ 0 ∞ ( x + 2 1 ) 2 + 4 3 1 d x = 3 2 ∫ 0 ∞ ( 3 2 x + 1 ) 2 + 1 1 d x = 3 1 ∫ 3 1 ∞ u 2 + 1 1 d u = 3 1 [ tan − 1 u ] 3 1 ∞ = 3 1 [ 2 π − 6 π ] = 3 3 π = 9 3 π Using identity: ∫ 0 ∞ f ( x ) d x = ∫ 0 ∞ x 2 f ( x 1 ) d x Let u = 3 2 x + 1 ⟹ d u = 3 2 d x
Therefore, a + b + c = 1 + 3 + 9 = 1 3 .