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Sequences and Series question

2025 · 29 Jan · Shift 2 · Q50
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  5. /2025 · 29 Jan · Shift 2 · Q50

Sequences and Series question

2025 · 29 Jan · Shift 2 · Q50

JEE MainMathematicsSequences and SeriesNumerical+4 / −1
Let a1,a2,…,a2024a_1, a_2, \ldots, a_{2024}a1​,a2​,…,a2024​ be an Arithmetic Progression such that a1+(a5+a10+a15+…+a2020)+a2024=2233a_1+\left(a_5+a_{10}+a_{15}+\ldots+a_{2020}\right)+a_{2024}=2233a1​+(a5​+a10​+a15​+…+a2020​)+a2024​=2233. Then a1+a2+a3+…+a2024a_1+a_2+a_3+\ldots+a_{2024}a1​+a2​+a3​+…+a2024​ is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 11132

Let the A.P. be an=a1+(n−1)d.a_n=a_1+(n-1)d.an​=a1​+(n−1)d. We are given a1+(a5+a10+a15+⋯+a2020)+a2024=2233.a_1+(a_5+a_{10}+a_{15}+\cdots+a_{2020})+a_{2024}=2233.a1​+(a5​+a10​+a15​+⋯+a2020​)+a2024​=2233.

We need to find S2024=a1+a2+⋯+a2024.S_{2024}=a_1+a_2+\cdots+a_{2024}.S2024​=a1​+a2​+⋯+a2024​.

1. Write the given sum in terms of a1a_1a1​ and ddd

The terms inside the bracket are a5,a10,a15,…,a2020.a_5, a_{10}, a_{15}, \ldots, a_{2020}.a5​,a10​,a15​,…,a2020​. These are terms whose indices are multiples of 555 from 555 to 202020202020.

Number of such terms: 20205=404.\frac{2020}{5}=404.52020​=404.

Now, a5k=a1+(5k−1)d,k=1,2,…,404.a_{5k}=a_1+(5k-1)d, \qquad k=1,2,\ldots,404.a5k​=a1​+(5k−1)d,k=1,2,…,404.

So, ∑k=1404a5k=∑k=1404(a1+(5k−1)d).\sum_{k=1}^{404} a_{5k} = \sum_{k=1}^{404}\bigl(a_1+(5k-1)d\bigr).∑k=1404​a5k​=∑k=1404​(a1​+(5k−1)d).

Hence, ∑k=1404a5k=404a1+d∑k=1404(5k−1).\sum_{k=1}^{404} a_{5k}=404a_1+d\sum_{k=1}^{404}(5k-1).∑k=1404​a5k​=404a1​+d∑k=1404​(5k−1).

Now, ∑k=1404(5k−1)=5∑k=1404k−404.\sum_{k=1}^{404}(5k-1)=5\sum_{k=1}^{404}k-404.∑k=1404​(5k−1)=5∑k=1404​k−404. Also, ∑k=1404k=404⋅4052=81810.\sum_{k=1}^{404}k=\frac{404\cdot 405}{2}=81810.∑k=1404​k=2404⋅405​=81810. Therefore, 5∑k=1404k−404=5(81810)−404=409050−404=408646.5\sum_{k=1}^{404}k-404=5(81810)-404=409050-404=408646.5∑k=1404​k−404=5(81810)−404=409050−404=408646.

Thus, a5+a10+⋯+a2020=404a1+408646d.a_5+a_{10}+\cdots+a_{2020}=404a_1+408646d.a5​+a10​+⋯+a2020​=404a1​+408646d.

Also, a2024=a1+2023d.a_{2024}=a_1+2023d.a2024​=a1​+2023d.

So the given condition becomes a1+(404a1+408646d)+(a1+2023d)=2233,a_1+(404a_1+408646d)+(a_1+2023d)=2233,a1​+(404a1​+408646d)+(a1​+2023d)=2233, that is, 406a_1+410669d=2233. \tag{1}

2. Express S2024S_{2024}S2024​

Sum of first 202420242024 terms of an A.P. is S2024=20242(2a1+2023d)=1012(2a1+2023d).S_{2024}=\frac{2024}{2}\bigl(2a_1+2023d\bigr)=1012(2a_1+2023d).S2024​=22024​(2a1​+2023d)=1012(2a1​+2023d).

So we need 2a1+2023d2a_1+2023d2a1​+2023d.

3. Relate equation (1) to 2a1+2023d2a_1+2023d2a1​+2023d

Observe that 410669=2023×203.410669=2023\times 203.410669=2023×203. Also, 406=2×203.406=2\times 203.406=2×203. Hence, 406a1+410669d=203(2a1+2023d).406a_1+410669d = 203(2a_1+2023d).406a1​+410669d=203(2a1​+2023d).

From (1), 203(2a1+2023d)=2233.203(2a_1+2023d)=2233.203(2a1​+2023d)=2233. So, 2a1+2023d=2233203=11.2a_1+2023d=\frac{2233}{203}=11.2a1​+2023d=2032233​=11.

4. Compute the required sum

Therefore, S2024=1012×11=11132.S_{2024}=1012\times 11=11132.S2024​=1012×11=11132.

5. Final answer

a1+a2+⋯+a2024=11132.a_1+a_2+\cdots+a_{2024}=11132.a1​+a2​+⋯+a2024​=11132.

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