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Question Number 54826 by rahul 19 last updated on 13/Feb/19

1) ∫_0 ^( 1)  ((1−x^7 )^(1/4) −(1−x^4 )^(1/7) )dx = ?  2) If f(x)=x^3 +3x+4 then the value of   ∫_(−1) ^( 1) f(x)dx + ∫_0 ^( 4) f^( −1) (x)dx = ?  3) ∫_(−π) ^( π) (1+cosx+cos2x+....+cos13x)(1+sinx+...+sin13x)dx=?  4) ∫_0 ^( 2)  ((√(x^3 +1)) + (x^2 +2x)^(1/3)  )dx =?  (This time func. are not inverse of each  other,right ?)

1)01((1x7)14(1x4)17)dx=?2)Iff(x)=x3+3x+4thenthevalueof11f(x)dx+04f1(x)dx=?3)ππ(1+cosx+cos2x+....+cos13x)(1+sinx+...+sin13x)dx=?4)02(x3+1+(x2+2x)13)dx=?(Thistimefunc.arenotinverseofeachother,right?)

Commented by maxmathsup by imad last updated on 12/Feb/19

2) we have ∫_(−1) ^1 f(x)dx =∫_(−1) ^1 (x^3  +3x+4)dx =[(x^4 /4) +(3/2)x^2  +4x]_(−1) ^1   =(1/4) +(3/2) +4 −(1/4) −(3/2) +4 =8   let find f^(−1) (x)  f(x)=y ⇔x^3  +3x +4 =y ⇔x^3  +3x+4−y =0   changement x =u+v give  u^3  +3uv(u+v) +v^3   +3(u+v) +4−y =0 ⇒  u^3  +v^3  +4−y + (u+v)(3uv+3) =0 ⇒u^3  +v^3  =y−4 and uv =−1 ⇒  so u^3  and v^3  are solution of the equation X^2 −(y−4)X −1 =0 ⇒  Δ =(y−4)^2  +4>0 ⇒X_1 =((y−4 +(√((y−4)^2  +4)))/2)  and  X_2 =((y−4 −(√((y−4)^2  )4)))/2)  ⇒x =^3 (√X_1 ) +(√X_2 )  ⇒f^(−1) (x)=(((x−4+(√((x−4)^2 +4)))/2))^(1/3)  +(((x−4 −(√((x−4)^2  +4)))/2))^(1/3)  ⇒  ∫_0 ^4  f^(−1) (x)dx =(1/((^3 (√2))))∫_0 ^4  (x−4+(√((x−4)^2  +4)))^(1/3) dx +(1/((^3 (√2))))∫_0 ^4 (x−4−(√((x−4)^2  +4)))^(1/3) dx  let I =∫_0 ^4 (x−4 +(√((x−4)^2  +4)))^(1/3) dx    chang .x−4 =2sh(t) give  I = ∫_(−argsh(2)) ^0  (2sh(t) +2 ch(t))^(1/3)  2ch(t)dt  =2(^3 (√2))  ∫_(−ln(2+(√5))) ^0  (sh(t)+ch(t)^(1/3)  ch(t)dt  ....be continued...  let A =∫_0 ^4  f^(−1) (x)dx  changement f^(−1) (x)= t give x=f(t) ⇒  A =∫_(f^(−1) (0)) ^(f^(−1) (4))   t f^′ (t)dt = [t f(t)]_(f^(−1) (0)) ^(f^(−1) (4))  −∫_(f^(−1) (0)) ^(f^(−1) (4))  f(t)dt  but f^(−1) (4) =1 −1 =0  and f^(−1) (0) =(((−4 +(√(20)))/2))^(1/3)  +(((−4−(√(20)))/2))^(1/3)   =(−2+(√5))^(1/3)  −(2+(√5))^(1/(3 ))   =α_0  ⇒  A =[tf(t)]_α_0  ^0  −∫_α_0  ^0  f(t)dt =−α_0 f(α_0 ) +∫_0 ^α_0  ( t^3  +3t+4)dt  =−α_0 f(α_0 ) +(α_0 ^3 /3) +(3/2) α_0 ^2  +4α_0   so the value of  ∫_0 ^4  f^(−1) (x) dx  is known .

2)wehave11f(x)dx=11(x3+3x+4)dx=[x44+32x2+4x]11=14+32+41432+4=8letfindf1(x)f(x)=yx3+3x+4=yx3+3x+4y=0changementx=u+vgiveu3+3uv(u+v)+v3+3(u+v)+4y=0u3+v3+4y+(u+v)(3uv+3)=0u3+v3=y4anduv=1sou3andv3aresolutionoftheequationX2(y4)X1=0Δ=(y4)2+4>0X1=y4+(y4)2+42andX2=y4(y4)2)42x=3X1+X2f1(x)=(x4+(x4)2+42)13+(x4(x4)2+42)1304f1(x)dx=1(32)04(x4+(x4)2+4)13dx+1(32)04(x4(x4)2+4)13dxletI=04(x4+(x4)2+4)13dxchang.x4=2sh(t)giveI=argsh(2)0(2sh(t)+2ch(t))132ch(t)dt=2(32)ln(2+5)0(sh(t)+ch(t)13ch(t)dt....becontinued...letA=04f1(x)dxchangementf1(x)=tgivex=f(t)A=f1(0)f1(4)tf(t)dt=[tf(t)]f1(0)f1(4)f1(0)f1(4)f(t)dtbutf1(4)=11=0andf1(0)=(4+202)13+(4202)13=(2+5)13(2+5)13=α0A=[tf(t)]α00α00f(t)dt=α0f(α0)+0α0(t3+3t+4)dt=α0f(α0)+α033+32α02+4α0sothevalueof04f1(x)dxisknown.

Commented by maxmathsup by imad last updated on 12/Feb/19

in general let find ∫_a ^b  f^(−1) (x)dx  chang.f^(−1) (x) =t give x =f(t) ⇒  ∫_a ^b  f^(−1) (x)dx =∫_(f^(−1) (a)) ^(f^(−1) (b))  t f^′ (t)dt =[t f(t)]_(f^(−1) (a)) ^(f^(−1) (b))  −∫_(f^(−1) (a)) ^(f^(−1) (b))  f(t)dt  =bf^(−1) (b)−af^(−1) (a) −∫_(f^(−1) (a)) ^(f^(−1) (b)) f(t)dt .

ingeneralletfindabf1(x)dxchang.f1(x)=tgivex=f(t)abf1(x)dx=f1(a)f1(b)tf(t)dt=[tf(t)]f1(a)f1(b)f1(a)f1(b)f(t)dt=bf1(b)af1(a)f1(a)f1(b)f(t)dt.

Commented by kaivan.ahmadi last updated on 12/Feb/19

y=f^(−1) (x)⇒f(y)=x⇒y^3 +3y+4=x,dx=(3y^2 +3)dy  if x=0⇒y=−1  if x=4⇒y=0 or −3   ∫_0 ^4 f^(−1) (x)dx=∫_(−1) ^0 y(3y^2 +3)dy=[((3y^4 )/4)+((3y^2 )/2)]_(−1) ^0 =−((3/4)+(3/2))=−(9/4)  is it true?

y=f1(x)f(y)=xy3+3y+4=x,dx=(3y2+3)dyifx=0y=1ifx=4y=0or304f1(x)dx=10y(3y2+3)dy=[3y44+3y22]10=(34+32)=94isittrue?

Commented by maxmathsup by imad last updated on 12/Feb/19

3) let simlify A(x)=(1+cosx +cos(2x)+...+cos(3x))(1+sinx +sin(2x)+...+sin(13x))  let S_n (x)=Σ_(k=0) ^n cos(kx) and W_n (x)=Σ_(k=0) ^n  sin(kx) ⇒ S_n  +iW_n   =Σ_(k=0) ^n  (e^(ix) )^k  =((1−(e^(ix) )^(n+1) )/(1−e^(ix) )) = ((1−e^(i(n+1)x) )/(1−cosx −isinx))  =((1−cos(n+1)x −isin(n+1)x)/(1−cosx −isinx)) =((2sin^2 (((n+1)x)/2)−2i sin((((n+1)x)/2))cos((((n+1)x)/2)))/(2 sin^2 ((x/2))−2i sin((x/2))cos((x/2))))  =((−i sin((((n+1)x)/2)) e^(i(((n+1)x)/2)) )/(−isin((x/2)) e^((ix)/2) )) =((sin((((n+1)x)/2))e^(i((nx)/2)) )/(sin((x/2))))  =((sin((((n+1)x)/2)))/(sin((x/2)))){ cos(((nx)/2)) +isin(((nx)/2))} ⇒   S_n (x) = ((sin((((n+1)x)/2)) cos(((nx)/2)))/(sin((x/2)))) and W_n (x)=((sin((((n+1)x)/2))sin(((nx)/2)))/(sin((x/2)))) ⇒  1+cosx +cos(2x)+...cos(13x)=((sin(7x)cos(((13x)/2)))/(sin((x/2)))) and   1+sinx +sin(2x)+...+sin(13x) =1+((sin(7x)sin(((13x)/2)))/(sin((x/2)))) ⇒  (...)×(...) =((sin(7x) cos(((13x)/2)))/(sin((x/2)))) +((sin^2 (7x)sin(13x))/(2sin^2 ((x/2))))  ⇒  ∫_(−π) ^π  A(x)dx = ∫_(−π) ^π  ((sin(7x)cos(((13x)/2)))/(sin((x/2))))dx  +∫_(−π) ^π   ((sin^2 (7x)sin(13x))/(2sin^2 ((x/2))))dx  ∫_(−π) ^π   ((sin(7x)cos(((13x)/2)))/(sin((x/2)))) dx =2 ∫_0 ^π    ((sin(7x)cos(((13x)/2)))/(sin((x/2))))dx (even function)  ∫_(−π) ^π   ((sin^2 (7x)sin(13x))/(2sin^2 ((x/2))))dx =0 ( odd function) ...be continued....

3)letsimlifyA(x)=(1+cosx+cos(2x)+...+cos(3x))(1+sinx+sin(2x)+...+sin(13x))letSn(x)=k=0ncos(kx)andWn(x)=k=0nsin(kx)Sn+iWn=k=0n(eix)k=1(eix)n+11eix=1ei(n+1)x1cosxisinx=1cos(n+1)xisin(n+1)x1cosxisinx=2sin2(n+1)x22isin((n+1)x2)cos((n+1)x2)2sin2(x2)2isin(x2)cos(x2)=isin((n+1)x2)ei(n+1)x2isin(x2)eix2=sin((n+1)x2)einx2sin(x2)=sin((n+1)x2)sin(x2){cos(nx2)+isin(nx2)}Sn(x)=sin((n+1)x2)cos(nx2)sin(x2)andWn(x)=sin((n+1)x2)sin(nx2)sin(x2)1+cosx+cos(2x)+...cos(13x)=sin(7x)cos(13x2)sin(x2)and1+sinx+sin(2x)+...+sin(13x)=1+sin(7x)sin(13x2)sin(x2)(...)×(...)=sin(7x)cos(13x2)sin(x2)+sin2(7x)sin(13x)2sin2(x2)ππA(x)dx=ππsin(7x)cos(13x2)sin(x2)dx+ππsin2(7x)sin(13x)2sin2(x2)dxππsin(7x)cos(13x2)sin(x2)dx=20πsin(7x)cos(13x2)sin(x2)dx(evenfunction)ππsin2(7x)sin(13x)2sin2(x2)dx=0(oddfunction)...becontinued....

Commented by Meritguide1234 last updated on 13/Feb/19

Answered by tanmay.chaudhury50@gmail.com last updated on 12/Feb/19

1)∫_0 ^1 (1−x^7 )^(1/4) dx−∫_0 ^1 (1−x^4 )^(1/7) dx  I_1 −I_2   ∫_0 ^1 (1−x^a )^(1/b) dx  x^a =sin^2 α  x=(sinα)^(2/a)   dx=(2/a)×(sinα)^((2/a)−1) (cosα)dα  ∫_0 ^(π/2) (1−sin^2 α)^(1/b) ×(2/a)×(sinα)^((2/a)−1) (cosα)dα  ∫_0 ^(π/2) (cosα)^(2/b) ×(2/a)×(sinα)^((2/a)−1) (cosα)dα  (2/a)∫_0 ^(π/2)  (sinα)^((2/a)−1) ×(cosα)^((2/b)+1) dα  (1/a)×2∫_0 ^(π/2) (sinα)^(2×(1/a)−1) (cosα)^(2((1/b)+1)−1) dα  formula  2∫_0 ^(π/2) (sinα)^(2p−1) (cosα)^(2q−1) dα  ((⌈(p)⌈q))/(⌈(p+q)))  so (1/a)×2∫_0 ^(π/2) (sinα)^(2×(1/a)−1) (cosα)^(2((1/b)+1)−1) dα  =(1/a)×((⌈((1/a))⌈((1/b)+1))/(⌈((1/a)+(1/b)+1)))  so  I_1 =(1/7)×((⌈((1/7))⌈((1/4)+1))/(⌈((1/4)+(1/7)+1))) [a=7   b=4]  I_2 =(1/4)×((⌈((1/4))⌈((1/7)+1))/(⌈((1/4)+(1/7)+1))) [a=4   b=7]  required ans=I_1 −I_2   formula ⌈(n+1)=n!  pls check upto this step...    I_1 −I_2 =(1/(⌈(((11)/(28))+1)))[(1/7)×⌈((1/7))×⌈((1/4)+1)−(1/4)×⌈((1/4))×⌈((1/7)+1)]  let a=(1/7)   b=(1/4)  ans=(1/(⌈(a+b+1)))×[a×⌈a)⌈(b+1)−b⌈(b)⌈(a+1)]  =(1/((a+b)!))×[a(a−1)!b!−b(b−1)!a!]  =(1/((a+b)!))[a!b!−b!a!]=0

1)01(1x7)14dx01(1x4)17dxI1I201(1xa)1bdxxa=sin2αx=(sinα)2adx=2a×(sinα)2a1(cosα)dα0π2(1sin2α)1b×2a×(sinα)2a1(cosα)dα0π2(cosα)2b×2a×(sinα)2a1(cosα)dα2a0π2(sinα)2a1×(cosα)2b+1dα1a×20π2(sinα)2×1a1(cosα)2(1b+1)1dαformula20π2(sinα)2p1(cosα)2q1dα(p)q)(p+q)so1a×20π2(sinα)2×1a1(cosα)2(1b+1)1dα=1a×(1a)(1b+1)(1a+1b+1)soI1=17×(17)(14+1)(14+17+1)[a=7b=4]I2=14×(14)(17+1)(14+17+1)[a=4b=7]requiredans=I1I2formula(n+1)=n!plscheckuptothisstep...I1I2=1(1128+1)[17×(17)×(14+1)14×(14)×(17+1)]leta=17b=14ans=1(a+b+1)×[a×a)(b+1)b(b)(a+1)]=1(a+b)!×[a(a1)!b!b(b1)!a!]=1(a+b)![a!b!b!a!]=0

Commented by mr W last updated on 12/Feb/19

f(x)=(1−x^7 )^(1/4) =y  ⇒x=(1−y^4 )^(1/7)   ⇒f^(−1) (x)=(1−x^4 )^(1/7) =g(x)  ⇒g(x) is inverse function of f(x), i.e.  g(f(x))=x  I_2 =∫_0 ^1 g(y)dy  let y=f(x)  ⇒g(y)=g(f(x))=x  dy=f′(x)dx  ⇒I_2 =∫_0 ^1 g(y)dy=∫_(g(0)) ^(g(1)) xf′(x)dx=∫_1 ^0 xf′(x)dx=−∫_0 ^1 xf′(x)dx  I_1 =∫_0 ^1 f(x)dx  I=I_1 −I_2 =∫_0 ^1 f(x)dx+∫_0 ^1 xf′(x)dx  =∫_0 ^1 [f(x)+xf′(x)]dx  =[xf(x)]_0 ^1   =0

f(x)=(1x7)14=yx=(1y4)17f1(x)=(1x4)17=g(x)g(x)isinversefunctionoff(x),i.e.g(f(x))=xI2=01g(y)dylety=f(x)g(y)=g(f(x))=xdy=f(x)dxI2=01g(y)dy=g(0)g(1)xf(x)dx=10xf(x)dx=01xf(x)dxI1=01f(x)dxI=I1I2=01f(x)dx+01xf(x)dx=01[f(x)+xf(x)]dx=[xf(x)]01=0

Commented by rahul 19 last updated on 12/Feb/19

Sir,I ′m new to this function!

Sir,Imnewtothisfunction!

Commented by rahul 19 last updated on 12/Feb/19

Commented by rahul 19 last updated on 12/Feb/19

this is sol^n given. Pl explain how   ∫_0 ^1 g(y)dy = ∫_1 ^0 xf ′(x)dx ?

thisissolngiven.Plexplainhow01g(y)dy=10xf(x)dx?

Commented by tanmay.chaudhury50@gmail.com last updated on 12/Feb/19

excellent sir...

excellentsir...

Commented by rahul 19 last updated on 13/Feb/19

Thank you sir!

Thankyousir!

Answered by mr W last updated on 12/Feb/19

Q(2)  f(x)=x^3 +3x+4  I_1 =∫_(−1) ^1 f(x)dx=∫_(−1) ^0 f(x)dx+∫_0 ^1 f(x)dx  I_2 =∫_0 ^4 f^(−1) (x)dx  let x=f(t)⇒t=f^(−1) (x)  dx=f′(t)dt  I_2 =∫_(f^(−1) (0)) ^(f^(−1) (4)) tf′(t)dt=∫_(−1) ^0 tf′(t)dt=∫_(−1) ^0 xf′(x)dx  I=I_1 +I_2 =∫_(−1) ^0 f(x)dx+∫_0 ^1 f(x)dx+∫_(−1) ^0 xf′(x)dx  =∫_(−1) ^0 [f(x)+xf′(x)]dx+∫_0 ^1 f(x)dx  =[xf(x)]_(−1) ^0 +∫_0 ^1 f(x)dx  =0+∫_0 ^1 f(x)dx  =∫_0 ^1 f(x)dx  =∫_0 ^1 (x^3 +3x+4)dx  =[(x^4 /4)+((3x^2 )/2)+4x]_0 ^1   =(1/4)+(3/2)+4  =((23)/4)   ⇒∫_(−1) ^( 1) f(x)dx + ∫_0 ^( 4) f^( −1) (x)dx =((23)/4)

Q(2)f(x)=x3+3x+4I1=11f(x)dx=10f(x)dx+01f(x)dxI2=04f1(x)dxletx=f(t)t=f1(x)dx=f(t)dtI2=f1(0)f1(4)tf(t)dt=10tf(t)dt=10xf(x)dxI=I1+I2=10f(x)dx+01f(x)dx+10xf(x)dx=10[f(x)+xf(x)]dx+01f(x)dx=[xf(x)]10+01f(x)dx=0+01f(x)dx=01f(x)dx=01(x3+3x+4)dx=[x44+3x22+4x]01=14+32+4=23411f(x)dx+04f1(x)dx=234

Commented by mr W last updated on 12/Feb/19

certainly one can also do like this:  I_1 =∫_(−1) ^1 f(x)dx=∫_(−1) ^1 (x^3 +3x+4)dx  =[(x^4 /4)+((3x^2 )/2)+4x]_(−1) ^1   =8  I_2 =∫_(f^(−1) (0)) ^(f^(−1) (4)) tf′(t)dt=∫_(−1) ^0 tf′(t)dt=∫_(−1) ^0 t(3t^2 +3)dt  =3∫_(−1) ^0 (t^3 +t)dt  =3[(t^4 /4)+(t^2 /2)]_(−1) ^0   =−3((1/4)+(1/2))  =−(9/4)  ⇒I=8−(9/4)=((23)/4)

certainlyonecanalsodolikethis:I1=11f(x)dx=11(x3+3x+4)dx=[x44+3x22+4x]11=8I2=f1(0)f1(4)tf(t)dt=10tf(t)dt=10t(3t2+3)dt=310(t3+t)dt=3[t44+t22]10=3(14+12)=94I=894=234

Commented by rahul 19 last updated on 13/Feb/19

yes , 2nd method works only when fun^c   is easy to integrate!

yes,2ndmethodworksonlywhenfunciseasytointegrate!

Answered by mr W last updated on 12/Feb/19

Q(3)  I=∫_(−π) ^( π) (1+cosx+cos2x+....+cos13x)(1+sinx+...+sin13x)dx  =∫_(−π) ^( π) (1+cosx+cos2x+....+cos13x)dx+∫_(−π) ^π (1+cosx+cos2x+....+cos13x)_(−−−−−−−−even−−−−−−−−−) (sinx+...+sin13x_(−−−−−−odd−−−−−) )dx  =∫_(−π) ^( π) (1+cosx+cos2x+....+cos13x)dx+0  =2∫_0 ^( π) (1+cosx+cos2x+....+cos13x)dx  =2(x+sin x+((sin 2x)/2)+...+((sin 13x)/(13)))_0 ^π   =2π

Q(3)I=ππ(1+cosx+cos2x+....+cos13x)(1+sinx+...+sin13x)dx=ππ(1+cosx+cos2x+....+cos13x)dx+ππ(1+cosx+cos2x+....+cos13x)even(sinx+...+sin13xodd)dx=ππ(1+cosx+cos2x+....+cos13x)dx+0=20π(1+cosx+cos2x+....+cos13x)dx=2(x+sinx+sin2x2+...+sin13x13)0π=2π

Commented by rahul 19 last updated on 13/Feb/19

thank you sir!

Answered by mr W last updated on 13/Feb/19

Q(4)  let I_2 =∫_0 ^2 (x^2 +2x)^(1/3) dx=∫_0 ^2 [(x+1)^2 −1]^(1/3) d(x+1)  =∫_1 ^3 (t^2 −1)^(1/3) dt  =∫_1 ^3 (x^2 −1)^(1/3) dx=∫_1 ^3 g(x)dx  with g(x)=(x^2 −1)^(1/3)       let f(x)=(√(x^3 +1))=y  x=(y^2 −1)^(1/3)   f^(−1) (x)=(x^2 −1)^(1/3) =g(x)    let I_1 =∫_0 ^2 (√(x^3 +1))dx=∫_0 ^2 f(x)dx  let x=g(t)  f(x)=f(g(t))=t  dx=g′(t)dt  I_1 =∫_0 ^2 f(x)dx=∫_(f(0)) ^(f(2)) tg′(t)dt=∫_1 ^3 tg′(t)dt  =∫_1 ^3 xg′(x)dx    I=I_1 +I_2 =∫_1 ^3 xg′(x)dx+∫_1 ^3 g(x)dx  =∫_1 ^3 [g(x)+xg′(x)]dx  =[xg(x)]_1 ^3   =3(3^2 −1)^(1/3) −1(1^2 −1)^(1/3)   =3×8^(1/3)   =3×2  =6  ⇒ ∫_0 ^( 2)  ((√(x^3 +1)) + (x^2 +2x)^(1/3)  )dx =6

Q(4)letI2=02(x2+2x)13dx=02[(x+1)21]13d(x+1)=13(t21)13dt=13(x21)13dx=13g(x)dxwithg(x)=(x21)13letf(x)=x3+1=yx=(y21)13f1(x)=(x21)13=g(x)letI1=02x3+1dx=02f(x)dxletx=g(t)f(x)=f(g(t))=tdx=g(t)dtI1=02f(x)dx=f(0)f(2)tg(t)dt=13tg(t)dt=13xg(x)dxI=I1+I2=13xg(x)dx+13g(x)dx=13[g(x)+xg(x)]dx=[xg(x)]13=3(321)131(121)13=3×813=3×2=602(x3+1+(x2+2x)13)dx=6

Commented by rahul 19 last updated on 14/Feb/19

Thanks sir!

Thankssir!

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