Elitmus
Exam
Numerical Ability
Sequence and Series
product of 11th and 15 th term of GP is 3.what would be the 5th root of the product of 25 terms of GP.
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- nth term of an gp is ar^n-1
so 11th term is ar^10 and 15th term is a^24
now ar^10*ar^14=3 i.e. a^2r^24=3.
now,
product of 25th term=a*ar*ar^2....*ar^24.
so, a^25r^(1+2+3...+24)
so,
a^25r^300.
because 1+2+3..+24=24*(1+24)/2=300.
now, 5th square root of product of 25th term is a^5*r^60 ie. a^2r^24*a^2r^24*ar^12 i.e 3*3*sq_root(3) = 9*sq.root(3). - 9 years agoHelpfull: Yes(26) No(1)
- Let the first 25 terms of GP be ar^-12 ,ar^-11 ,ar^-10,............ ,a,ar,.............. ,ar^11 ,ar^12
Therefore Product of 25 terms of GP is a^25 .
5th root of product of first 25 terms is (a^25)^1/5 = a^5.
11th term in above series is ar^-2
15th term is ar^2
So product of 11th and 15th terms is a^2 =3
Therfore answer is a^5 = a^2 * a^2 *a
=3 * 3* sq.rt(3)
=9 {sq.rt (3)} - 9 years agoHelpfull: Yes(8) No(0)
- let first term be a and common difference be d.
so ar^10*ar^14=3 now as 3 is either equal to 1*3 or3*1 ,hence ar^110=1 and ar^14=3
dividing ar^14/ar^10=r^4=3 i.e r=3 ^1/4 a=1/r^10=1/3^10/4=1/3^5/2
now a*ar*ar^2..........ar^24=product of first 25 terms=a^25*r^1+2+3+4......+24=a^25*r^300=(1/3^5/2)^25*3^300/4 - 9 years agoHelpfull: Yes(1) No(0)
- let the terms be a , ar, ar^2.......................................
now the product of 11th and 15th term will be (ar^10*ar14) = a^2r^24 = 3
now product of 25 terms (a*ar*ar^2.............................*ar^24)=a^25*r^(0+1+2+3.................................+24)
now(0+1+2.........24)= 300
therefore it becomes a^25*r^300
now its fifth root=( a^25*r^60)^1/5= a^5*r^60
now (a^5*r^12)^2=a^10*r^120
now,(a^2*r^24)^5=3^5=a^10*r^120
therefore sqrt(a^10*r^120)= a^5*r^60=sqrt(3^5)
=9sqrt(3) Ans
- 9 years agoHelpfull: Yes(0) No(0)
- a^2*r^24=3 a*r^12=3
product of 25 terms is a^5*r^300
so 5th root is 3^0.5 - 9 years agoHelpfull: Yes(0) No(0)
- ANS 9*sqroot(3)
formula of GP is (a.r^n-1)
here (11th * 15th)term=3
ar^10*ar^14=3
a^2r^24=3--------------------------------------------1
and here 25 term not 25th term(25 term and 25th term totaly different)
a*ar*ar^2*ar^3*...............*ar^24
a^25 r^(1+2+3+4+...................+24)
by formula of AP we no
1+2+3+4+.....................+n=n(n+1)/2
so
a^25 r^(1+2+3+4+...................+24)
(1+2+3+4+...................+24)
here n=24
24(24+1)/2=12*25=300
a^25 r^300
but in qustion 5th root of the product of 25 terms of GP
a^5r^60
(by this equation a^2r^24=3--------------------------------------------1)
a^5r^60=a^2r^24 a^2r^24 a r^12(we can write)
3 3 sqr root of 3
ANS 9*sqroot(3)
- 9 years agoHelpfull: Yes(0) No(0)
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