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let-I-n-0-dx-1-x-3-n-find-a-relation-etween-I-n-and-I-n-1-2-calculate-I-1-and-I-2-




Question Number 40159 by maxmathsup by imad last updated on 16/Jul/18
let I_n = ∫_0 ^∞     (dx/((1+x^3 )^n ))  find a relation etween I_n  and I_(n+1)   2) calculate I_(1 )  and I_2
letIn=0dx(1+x3)nfindarelationetweenInandIn+12)calculateI1andI2
Commented by prof Abdo imad last updated on 17/Jul/18
1) we have  I_n = ∫_0 ^∞     ((1+x^3 )/((1+x^3 )^(n+1) ))dx  =I_(n+1)   + ∫_0 ^∞    (x^3 /((1+x^3 )^(n+1) ))dx  by parts  ∫_0 ^∞     (x^3 /((1+x^3 )^3 ))dx =(1/3)∫_0 ^∞   x(((3x^2 )/((1+x^3 )^(n+1) )))dx  =(1/3)∫_0 ^∞   x ( 3x^2  (1+x^3 )^(−n−1) )dx  =(1/3)[−(x/n)(1+x^3 )^(−n) ]_0 ^(+∞)  −(1/3)∫_0 ^∞   1 (−(1/n))(1/((1+x^3 )^n ))dx  = (1/(3n))  I_n  ⇒ I_n = I_(n+1)    +(1/(3n)) I_n  ⇒  (1−(1/(3n))) I_n  = I_(n+1)   ⇒ I_(n+1) =((3n−1)/(3n)) I_n   2) we have I_1 = ∫_0 ^∞     (dx/(1+x^3 ))  =_(x^ =t^(1/3) )     ∫_0 ^∞     (1/(1+t)) (1/3)t^((1/3)−1) dt =(1/3) ∫_0 ^∞     (t^((1/3)−1) /(1+t))dt  =(1/3) (π/(sin((π/3)))) =(π/(3.((√3)/2))) = ((2π)/(3(√3)))  I_2 = (2/3) I_1 = (2/3) ((2π)/(3(√3))) = ((4π)/(9(√3)))  .
1)wehaveIn=01+x3(1+x3)n+1dx=In+1+0x3(1+x3)n+1dxbyparts0x3(1+x3)3dx=130x(3x2(1+x3)n+1)dx=130x(3x2(1+x3)n1)dx=13[xn(1+x3)n]0+1301(1n)1(1+x3)ndx=13nInIn=In+1+13nIn(113n)In=In+1In+1=3n13nIn2)wehaveI1=0dx1+x3=x=t13011+t13t131dt=130t1311+tdt=13πsin(π3)=π3.32=2π33I2=23I1=232π33=4π93.

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