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y-x-1-x-a-a-x-t-t-t-t-t-dt-x-a-a-gt-0-y-x-gt-0-Find-y-2a-




Question Number 1625 by 112358 last updated on 27/Aug/15
y(x)=(1/(x−a))∫_a ^x (√(t+(√(t+(√(t+(√(t+(√(t+...))))))))))dt  x≠a, a>0,y(x)>0.  Find  y(2a).
y(x)=1xaaxt+t+t+t+t+dtxa,a>0,y(x)>0.Findy(2a).
Answered by Rasheed Soomro last updated on 28/Aug/15
y(x)=(1/(x−a))∫_a ^( x) ( (√(t+(√(t+(√(t+(√(t+(√(t+...)))))))))) )dt  Let   (√(t+(√(t+(√(t+(√(t+(√(t+...))))))))))=u  Squaring both sides:            t+(√(t+(√(t+(√(t+(√(t+...))))))))=u^2           t+u=u^(2   ) ⇒  u^2 −u−t=0           u=((1±(√(1+4t)))/2)  y(x)=(1/(x−a)) ∫_a ^( x) (((1±(√(1+4t)))/2) )dt            =(1/(2(x−a)))∫_a ^( x) (1±(√(1+4t)))dt           =(1/(2(x−a)))[t±(1/6)(1+4t)^(3/2)  ]_a ^x    =(1/(2(x−a)))([x±((√((1+4x)^3 ))/6)]−[a±((√((1+4a)^3 ))/6)])  =(1/(2(x−a)))×((6(x−a)±(√((1+4x)^3 )) ∓(√((1+4a)^3 )) )/6)  =((6(x−a)±((√((1+4x)^3 )) −(√((1+4a)^3 )) ))/(12(x−a)))  y(x)=(1/2)±(((√((1+4x)^3 )) −(√((1+4a)^3 )))/(12(x−a)))  y(2a)=(1/2)±(((√((1+4(2a))^3 )) −(√((1+4a)^3 )))/(12((2a)−a)))             =(1/2)±(((√((1+8a)^3 )) −(√((1+4a)^3 )))/(12a))
y(x)=1xaax(t+t+t+t+t+)dtLett+t+t+t+t+=uSquaringbothsides:t+t+t+t+t+=u2t+u=u2u2ut=0u=1±1+4t2y(x)=1xaax(1±1+4t2)dt=12(xa)ax(1±1+4t)dt=12(xa)[t±16(1+4t)32]ax=12(xa)([x±(1+4x)36][a±(1+4a)36])=12(xa)×6(xa)±(1+4x)3(1+4a)36=6(xa)±((1+4x)3(1+4a)3)12(xa)y(x)=12±(1+4x)3(1+4a)312(xa)y(2a)=12±(1+4(2a))3(1+4a)312((2a)a)=12±(1+8a)3(1+4a)312a
Commented by 123456 last updated on 28/Aug/15
∫_a ^x 1±(√(1+4t))dt=∫_a ^x dt±∫_a ^x (√(1+4t))dt  ∫_a ^x dt=x−a  ∫_a ^x (√(1+4t))dt  u=1+4t⇒du=4dt  t=a⇒u=1+4a  t=x⇒u=1+4x  ∫_a ^x (√(1+4t))dt=(1/4)∫_(1+4a) ^(1+4x) (√u)du=[((1/4)/(3/2))(√u^3 )]_(1+4a) ^(1+4x)                        =(((√((1+4x)^3 ))−(√((1+4a)^3 )))/6)  ∫_a ^x 1±(√(1+4t))dt=x−a±(((√((1+4x)^3 ))−(√((1+4a)^3 )))/6)
xa1±1+4tdt=xadt±xa1+4tdtxadt=xaxa1+4tdtu=1+4tdu=4dtt=au=1+4at=xu=1+4xxa1+4tdt=141+4x1+4audu=[1/43/2u3]1+4a1+4x=(1+4x)3(1+4a)36xa1±1+4tdt=xa±(1+4x)3(1+4a)36
Commented by 123456 last updated on 28/Aug/15
y(x)=(1/(2(x−a)))×((6(x−a)±(√((1+4x)^3 ))∓(√((1+4a)^3 )))/6)
y(x)=12(xa)×6(xa)±(1+4x)3(1+4a)36
Commented by Rasheed Soomro last updated on 28/Aug/15
Thanks to mention my calculation mistake.I am going to  correct it.
Thankstomentionmycalculationmistake.Iamgoingtocorrectit.
Commented by 123456 last updated on 28/Aug/15
lim_(x→a)  y(x) is like [(d/dx)∫f(x)dx]_(x=a)  :)  lim_(x→a) ((∫_a ^x f(x)dx)/(x−a))=lim_(x→a) ((F(x)−F(a))/(x−a))=f(a)
limxay(x)islike[ddxf(x)dx]x=a:)limxaxaf(x)dxxa=limxaF(x)F(a)xa=f(a)
Commented by 112358 last updated on 29/Aug/15
I created this question having the  formula for the average value  of a function in mind:                  y_(avg) =(1/(b−a))∫_a ^b f(x)dx .                  y_(avg) =((F(b)−F(a))/(b−a))    (∗)  (∗) resembles the relation given  by the mean value theorem which  states that ∃c∈[a,b] such that            (d/dx)f(x)∣_(x=c) =((f(a)−f(b))/(b−a))  given that f(x) is continuous   ∀x∈[a,b] and differentiable  ∀x∈(a,b).
Icreatedthisquestionhavingtheformulafortheaveragevalueofafunctioninmind:yavg=1baabf(x)dx.yavg=F(b)F(a)ba()()resemblestherelationgivenbythemeanvaluetheoremwhichstatesthatc[a,b]suchthatddxf(x)x=c=f(a)f(b)bagiventhatf(x)iscontinuousx[a,b]anddifferentiablex(a,b).
Commented by 123456 last updated on 29/Aug/15
or mean value theorem for integrails  if f is continuous and integrable into [a,b]  them ∃ξ∈[a,b]  ∫_a ^b f(x)dx=f(ξ)(b−a)  so for some ξ∈[a,b]  y_(avg) =(1/(b−a))∫_a ^b f(x)dx=f(ξ)
ormeanvaluetheoremforintegrailsiffiscontinuousandintegrableinto[a,b]themξ[a,b]baf(x)dx=f(ξ)(ba)soforsomeξ[a,b]yavg=1babaf(x)dx=f(ξ)
Commented by 112358 last updated on 29/Aug/15
Nice. Thanks!
Nice.Thanks!Nice.Thanks!

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