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Sequences and Series question

2022 · 26 Jun · Shift 2 · Q44
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Sequences and Series question

2022 · 26 Jun · Shift 2 · Q44

JEE MainMathematicsSequences and SeriesNumerical+4 / −1
If a1 (> 0), a2, a3, a4, a5 are in a G.P., a2 + a4 = 2a3 + 1 and 3a2 + a3 = 2a4, then a2 + a4 + 2a5 is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 40

  1. Set up the G.P. terms

Since a1,a2,a3,a4,a5a_1, a_2, a_3, a_4, a_5a1​,a2​,a3​,a4​,a5​ are in G.P. and a1>0a_1>0a1​>0, let a1=a, a2=ar, a3=ar2, a4=ar3, a5=ar4a_1=a,\, a_2=ar,\, a_3=ar^2,\, a_4=ar^3,\, a_5=ar^4a1​=a,a2​=ar,a3​=ar2,a4​=ar3,a5​=ar4 for some a>0a>0a>0 and common ratio rrr.

We are given: a2+a4=2a3+1a_2+a_4=2a_3+1a2​+a4​=2a3​+1 3a2+a3=2a43a_2+a_3=2a_43a2​+a3​=2a4​

Substitute the G.P. terms.

  1. Use the second equation first

From 3a2+a3=2a43a_2+a_3=2a_43a2​+a3​=2a4​ we get 3ar+ar2=2ar33ar+ar^2=2ar^33ar+ar2=2ar3 Since a≠0a\neq 0a=0, 3r+r2=2r33r+r^2=2r^33r+r2=2r3 r(3+r)=2r3r(3+r)=2r^3r(3+r)=2r3 As a2,a3,a4a_2,a_3,a_4a2​,a3​,a4​ are G.P. terms, taking r≠0r\neq 0r=0, divide by rrr: 3+r=2r23+r=2r^23+r=2r2 2r2−r−3=02r^2-r-3=02r2−r−3=0 Factorizing, (2r−3)(r+1)=0(2r-3)(r+1)=0(2r−3)(r+1)=0 So, r=32orr=−1r=\frac32 \quad \text{or} \quad r=-1r=23​orr=−1

  1. Use the first equation

From a2+a4=2a3+1a_2+a_4=2a_3+1a2​+a4​=2a3​+1 we get ar+ar3=2ar2+1ar+ar^3=2ar^2+1ar+ar3=2ar2+1 a(r+r3−2r2)=1a(r+r^3-2r^2)=1a(r+r3−2r2)=1 ar(r2−2r+1)=1ar(r^2-2r+1)=1ar(r2−2r+1)=1 ar(r−1)2=1ar(r-1)^2=1ar(r−1)2=1

Now test the two possible values of rrr.


Case 1: r=−1r=-1r=−1

Then ar(r−1)2=a(−1)(−2)2=−4aar(r-1)^2=a(-1)(-2)^2=-4aar(r−1)2=a(−1)(−2)2=−4a So the equation becomes −4a=1-4a=1−4a=1 which gives a=−14a=-\frac14a=−41​ But this contradicts a1=a>0a_1=a>0a1​=a>0.

So this case is rejected.


Case 2: r=32r=\frac32r=23​

Then ar(r−1)2=a(32)(12)2=a⋅32⋅14=3a8ar(r-1)^2=a\left(\frac32\right)\left(\frac12\right)^2= a\cdot \frac32\cdot \frac14=\frac{3a}{8}ar(r−1)2=a(23​)(21​)2=a⋅23​⋅41​=83a​ Hence 3a8=1\frac{3a}{8}=183a​=1 a=83a=\frac{8}{3}a=38​

Thus, a2=ar=83⋅32=4a_2=ar=\frac{8}{3}\cdot \frac32=4a2​=ar=38​⋅23​=4 a3=ar2=83⋅94=6a_3=ar^2=\frac{8}{3}\cdot \frac94=6a3​=ar2=38​⋅49​=6 a4=ar3=83⋅278=9a_4=ar^3=\frac{8}{3}\cdot \frac{27}{8}=9a4​=ar3=38​⋅827​=9 a5=ar4=83⋅8116=272a_5=ar^4=\frac{8}{3}\cdot \frac{81}{16}=\frac{27}{2}a5​=ar4=38​⋅1681​=227​

  1. Find the required value

We need a2+a4+2a5a_2+a_4+2a_5a2​+a4​+2a5​ Substitute: 4+9+2(272)=4+9+27=404+9+2\left(\frac{27}{2}\right)=4+9+27=404+9+2(227​)=4+9+27=40

  1. Final answer

40\boxed{40}40​

The derived answer matches the stored correct answer.

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