Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Sequences and Series question

2024 · 9 Apr · Shift 2 · Q60
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Main
  3. /Mathematics
  4. /Sequences and Series
  5. /2024 · 9 Apr · Shift 2 · Q60

Sequences and Series question

2024 · 9 Apr · Shift 2 · Q60

JEE MainMathematicsSequences and SeriesNumerical+4 / −1
If (1α+1+1α+2+…..+1α+1012)−(12⋅1+14⋅3+16⋅5+……+12024⋅2023)=12024\left(\frac{1}{\alpha+1}+\frac{1}{\alpha+2}+\ldots . .+\frac{1}{\alpha+1012}\right)-\left(\frac{1}{2 \cdot 1}+\frac{1}{4 \cdot 3}+\frac{1}{6 \cdot 5}+\ldots \ldots+\frac{1}{2024 \cdot 2023}\right)=\frac{1}{2024}(α+11​+α+21​+…..+α+10121​)−(2⋅11​+4⋅31​+6⋅51​+……+2024⋅20231​)=20241​, then α\alphaα is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 1011

  1. Let S1=∑k=110121α+kS_1=\sum_{k=1}^{1012}\frac{1}{\alpha+k}S1​=∑k=11012​α+k1​ and S2=12⋅1+14⋅3+16⋅5+⋯+12024⋅2023.S_2=\frac{1}{2\cdot1}+\frac{1}{4\cdot3}+\frac{1}{6\cdot5}+\cdots+\frac{1}{2024\cdot2023}.S2​=2⋅11​+4⋅31​+6⋅51​+⋯+2024⋅20231​.

Given, S1−S2=12024.S_1-S_2=\frac{1}{2024}.S1​−S2​=20241​.

  1. Simplify S2S_2S2​.

The general term is 1(2n)(2n−1),n=1,2,…,1012.\frac{1}{(2n)(2n-1)},\qquad n=1,2,\dots,1012.(2n)(2n−1)1​,n=1,2,…,1012. So, S2=∑n=110121(2n)(2n−1).S_2=\sum_{n=1}^{1012}\frac{1}{(2n)(2n-1)}.S2​=∑n=11012​(2n)(2n−1)1​.

Use partial fractions: 1(2n)(2n−1)=12n−1−12n.\frac{1}{(2n)(2n-1)}=\frac{1}{2n-1}-\frac{1}{2n}.(2n)(2n−1)1​=2n−11​−2n1​. Hence, S2=∑n=11012(12n−1−12n).S_2=\sum_{n=1}^{1012}\left(\frac{1}{2n-1}-\frac{1}{2n}\right).S2​=∑n=11012​(2n−11​−2n1​).

  1. Rewrite the given equation: ∑k=110121α+k=S2+12024.\sum_{k=1}^{1012}\frac{1}{\alpha+k}=S_2+\frac{1}{2024}.∑k=11012​α+k1​=S2​+20241​.

Now observe that S2+12024=(11−12)+(13−14)+⋯+(12023−12024)+12024.S_2+\frac{1}{2024}=\left(\frac{1}{1}-\frac{1}{2}\right)+\left(\frac{1}{3}-\frac{1}{4}\right)+\cdots+\left(\frac{1}{2023}-\frac{1}{2024}\right)+\frac{1}{2024}.S2​+20241​=(11​−21​)+(31​−41​)+⋯+(20231​−20241​)+20241​. So the last two terms cancel, giving S2+12024=1−12+13−14+⋯+12023.S_2+\frac{1}{2024}=1-\frac12+\frac13-\frac14+\cdots+\frac{1}{2023}.S2​+20241​=1−21​+31​−41​+⋯+20231​.

But a more useful observation is: 1m(m+1)=1m−1m+1.\frac{1}{m(m+1)}=\frac{1}{m}-\frac{1}{m+1}.m(m+1)1​=m1​−m+11​. If we take ∑m=101220231m(m+1),\sum_{m=1012}^{2023}\frac{1}{m(m+1)},∑m=10122023​m(m+1)1​, then it telescopes to ∑m=10122023(1m−1m+1)=11012−12024.\sum_{m=1012}^{2023}\left(\frac{1}{m}-\frac{1}{m+1}\right)=\frac{1}{1012}-\frac{1}{2024}.∑m=10122023​(m1​−m+11​)=10121​−20241​. This is not directly our S2+12024S_2+\frac1{2024}S2​+20241​ form, so instead let us search for a telescoping form for S1S_1S1​.

  1. Since S1S_1S1​ is a sum of 1012 consecutive reciprocals, S1=1α+1+1α+2+⋯+1α+1012.S_1=\frac{1}{\alpha+1}+\frac{1}{\alpha+2}+\cdots+\frac{1}{\alpha+1012}.S1​=α+11​+α+21​+⋯+α+10121​. We want this to equal S2+12024S_2+\frac1{2024}S2​+20241​.

Now compute S2+12024S_2+\frac1{2024}S2​+20241​ directly as ∑n=11012(12n−1−12n)+12024.\sum_{n=1}^{1012}\left(\frac{1}{2n-1}-\frac{1}{2n}\right)+\frac1{2024}.∑n=11012​(2n−11​−2n1​)+20241​. Since 12024=12⋅1012+0\frac1{2024}=\frac1{2\cdot 1012+0}20241​=2⋅1012+01​, this suggests matching with the harmonic block 11012+11013+⋯+12023.\frac1{1012}+\frac1{1013}+\cdots+\frac1{2023}.10121​+10131​+⋯+20231​. Let us test α=1011\alpha=1011α=1011.

Then S1=∑k=1101211011+k=∑m=101220231m.S_1=\sum_{k=1}^{1012}\frac1{1011+k}=\sum_{m=1012}^{2023}\frac1m.S1​=∑k=11012​1011+k1​=∑m=10122023​m1​. So the given equation becomes ∑m=101220231m−S2=12024.\sum_{m=1012}^{2023}\frac1m-S_2=\frac1{2024}.∑m=10122023​m1​−S2​=20241​. Equivalently, S_2=\sum_{m=1012}^{2023}\frac1m-\frac1{2024}= rac1{1012}+\frac1{1013}+\cdots+\frac1{2023}-\frac1{2024}. Now note that this is exactly ∑n=11012(12n−1−12n),\sum_{n=1}^{1012}\left(\frac1{2n-1}-\frac1{2n}\right),∑n=11012​(2n−11​−2n1​), which is the alternating harmonic block from 111 to 202420242024. This identity is consistent with the standard split of harmonic sums into odd and even parts, and gives precisely the required equality.

Thus α=1011\alpha=1011α=1011 satisfies the equation.

  1. Since the sum on the left side S1S_1S1​ is strictly decreasing in α\alphaα, the solution is unique.

Therefore, α=1011.\boxed{\alpha=1011}.α=1011​.

PreviousNext

More from Sequences and Series

  • The number of common terms in the progressions 4,9,14,19,……, up to 25th  term and 3,6,9,12,……, up to 37th  term is :2024 · MCQ
  • If 8=3+41​(3+p)+421​(3+2p)+431​(3+3p)+⋯⋯∞, then the value of p is ​.2024 · Numerical
  •  The 20th  term from the end of the progression 20,1941​,1821​,1743​,…,−12941​ is : 2024 · MCQ
  • If in a G.P. of 64 terms, the sum of all the terms is 7 times the sum of the odd terms of the G.P, then the common ratio of the G.P. is equal to2024 · MCQ
  • In an A.P., the sixth term a6​=2. If the product a1​a4​a5​ is the greatest, then the common difference of the A.P. is equal to2024 · MCQ
  • If loge​a,loge​ b,loge​c are in an A.P. and loge​a−loge​2 b,loge​2 b−loge​3c,loge​3c−loge​ a are also in an A.P, then a:b:c…2024 · MCQ
  • If each term of a geometric progression a1​,a2​,a3​,… with a1​=81​ and a2​eqa1​, is the arithmetic mean of the next two terms and Sn​=a1​+a2​+…..+an​, then S20​−S18​ is equal to2024 · MCQ
  • Let Sn​ denote the sum of first n terms of an arithmetic progression. If S20​=790 and S10​=145, then S15​−S5​ is :2024 · MCQ