∫ 0 1 x 2 − x + 1 arcsin x d x
If the integral above is equal to n π 2 , find n .
This Question is Part of My Mathematics Set
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Rather than substitution directly apply a+b-x property and use the formula of arcsina+arcsinb to get the integral of 1/x^2-x+1
I = ∫ 0 1 1 − x ( 1 − x ) arcsin ( x ) d x I = ∫ 0 1 1 − x ( 1 − x ) arcsin ( 1 − x ) d x 2 I = ∫ 0 1 1 − x ( 1 − x ) arcsin ( x ) + arcsin ( 1 − x ) d x arcsin ( x ) + arcsin ( 1 − x ) = arcsin ( sin ( arcsin ( x ) + arcsin ( 1 − x ) ) ) = arcsin ( sin ( arcsin ( x ) ) cos ( arcsin ( 1 − x ) ) + sin ( arcsin ( 1 − x ) ) cos ( arcsin ( x ) ) ) = arcsin ( x + 1 − x ) = 2 π I = 4 π ∫ 0 1 1 − x ( 1 − x ) 1 d x = 4 π ∫ 0 1 ( x − 1 / 2 ) 2 + 3 / 4 1 d x = 4 π 3 2 arctan ( 3 2 ( x − 1 ) ) ∣ 0 1 = 6 3 π 2 = 1 0 8 π 2
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Similar method with Akhil Jain 's
I = ∫ 0 1 x 2 − x + 1 arcsin x d x = ∫ 0 2 π sin 4 θ − sin 2 θ + 1 2 θ sin θ cos θ d θ = 2 1 ∫ 0 2 π sin 4 θ − sin 2 θ + 1 2 θ sin θ cos θ + cos 4 θ − cos 2 θ + 1 2 ( 2 π − θ ) cos θ sin θ d θ = 2 1 ∫ 0 2 π sin 4 θ − sin 2 θ + 1 2 θ sin θ cos θ + sin 4 θ − sin 2 θ + 1 ( π − 2 θ ) sin θ cos θ d θ = 2 π ∫ 0 2 π sin 4 θ − sin 2 θ + 1 sin θ cos θ d θ = 2 π ∫ 0 1 u 4 − u 2 + 1 u d u = 4 π ∫ 0 1 t 2 − t + 1 1 d t = 4 π ∫ 0 1 ( t − 2 1 ) 2 + 4 3 1 d t = 3 π ∫ 0 1 3 4 ( t − 2 1 ) 2 + 1 1 d t = 2 3 π ∫ − 3 1 3 1 v 2 + 1 1 d v = 2 3 π tan − 1 v ∣ ∣ ∣ ∣ − 3 1 3 1 = 6 3 π 2 = 1 0 8 π 2 Let sin θ = x , sin 2 θ = x , d x = 2 sin θ cos θ d θ Using ∫ a b f ( x ) d x = ∫ a b f ( a + b − x ) d x Let u = sin θ , d u = cos θ d θ Let t = u 2 , d t = 2 u d u Let v = 3 2 ( t − 2 1 ) , d v = 3 2 d t
⟹ n = 1 0 8