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Sequences and Series question

2022 · 27 Jul · Shift 1 · Q29
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  5. /2022 · 27 Jul · Shift 1 · Q29

Sequences and Series question

2022 · 27 Jul · Shift 1 · Q29

JEE MainMathematicsSequences and SeriesMCQ+4 / −1
Suppose a1,a2,…,ana_{1}, a_{2}, \ldots, a_{n}a1​,a2​,…,an​, .. be an arithmetic progression of natural numbers. If the ratio of the sum of first five terms to the sum of first nine terms of the progression is 5:175: 175:17 and , 110<a15<120110 \lt {a_{15}} \lt 120110<a15​<120, then the sum of the first ten terms of the progression is equal to
  1. A
    290
  2. B
    380
  3. C
    460
  4. D
    510
View written solutionFree

Correct answer: B

  1. Let the arithmetic progression have first term aaa and common difference ddd.

    Then the sum of first nnn terms is Sn=n2[2a+(n−1)d].S_n = \frac{n}{2}\left[2a+(n-1)d\right].Sn​=2n​[2a+(n−1)d].

  2. Use the given ratio: S5S9=517.\frac{S_5}{S_9}=\frac{5}{17}.S9​S5​​=175​.

    Now, S5=52(2a+4d)=5(a+2d),S_5=\frac{5}{2}(2a+4d)=5(a+2d),S5​=25​(2a+4d)=5(a+2d), S9=92(2a+8d)=9(a+4d).S_9=\frac{9}{2}(2a+8d)=9(a+4d).S9​=29​(2a+8d)=9(a+4d).

    So, 5(a+2d)9(a+4d)=517.\frac{5(a+2d)}{9(a+4d)}=\frac{5}{17}.9(a+4d)5(a+2d)​=175​.

  3. Cancel 555 from both sides: a+2d9(a+4d)=117.\frac{a+2d}{9(a+4d)}=\frac{1}{17}.9(a+4d)a+2d​=171​.

    Cross-multiplying, 17(a+2d)=9(a+4d).17(a+2d)=9(a+4d).17(a+2d)=9(a+4d).

    17a+34d=9a+36d17a+34d=9a+36d17a+34d=9a+36d 8a=2d8a=2d8a=2d d=4a.d=4a.d=4a.

  4. Since all terms are natural numbers, take a=k,d=4ka=k, \quad d=4ka=k,d=4k for some natural number kkk.

  5. Now find a15a_{15}a15​: a15=a+14d=k+14(4k)=57k.a_{15}=a+14d=k+14(4k)=57k.a15​=a+14d=k+14(4k)=57k.

    Given 110<a15<120,110<a_{15}<120,110<a15​<120, so 110<57k<120.110<57k<120.110<57k<120.

    Checking natural number values of kkk:

    • If k=1k=1k=1, then 57k=5757k=5757k=57 (too small)
    • If k=2k=2k=2, then 57k=11457k=11457k=114 (fits)
    • If k≥3k\ge 3k≥3, then 57k≥17157k\ge 17157k≥171 (too large)

    Hence, k=2.k=2.k=2.

    Therefore, a=2,d=8.a=2, \quad d=8.a=2,d=8.

  6. Now compute the sum of first 10 terms: S10=102[2a+9d]S_{10}=\frac{10}{2}[2a+9d]S10​=210​[2a+9d] =5[2(2)+9(8)]=5[2(2)+9(8)]=5[2(2)+9(8)] =5(4+72)=5(4+72)=5(4+72) =5×76=380.=5\times 76=380.=5×76=380.

  7. Therefore, the required sum is 380.\boxed{380}. 380​.

  8. Option check:

    • A: 290290290 ❌
    • B: 380380380 ✅
    • C: 460460460 ❌
    • D: 510510510 ❌
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