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Sequences and Series question

2024 · 27 Jan · Shift 1 · Q46
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Sequences and Series question

2024 · 27 Jan · Shift 1 · Q46

JEE MainMathematicsSequences and SeriesMCQ+4 / −1
The number of common terms in the progressions 4,9,14,19,……4,9,14,19, \ldots \ldots4,9,14,19,……, up to 25th 25^{\text {th }}25th  term and 3,6,9,12,……3,6,9,12, \ldots \ldots3,6,9,12,……, up to 37th 37^{\text {th }}37th  term is :
  1. A
    9
  2. B
    8
  3. C
    5
  4. D
    7
View written solutionFree

Correct answer: D

  1. Write the two progressions explicitly as APs

    • First progression: 4,9,14,19,…4,9,14,19,\ldots4,9,14,19,…

      This is an AP with: a1=4,d1=5a_1=4,\quad d_1=5a1​=4,d1​=5 Its nnn-th term is Tn=4+(n−1)5=5n−1T_n=4+(n-1)5=5n-1Tn​=4+(n−1)5=5n−1

    • Second progression: 3,6,9,12,…3,6,9,12,\ldots3,6,9,12,…

      This is an AP with: a2=3,d2=3a_2=3,\quad d_2=3a2​=3,d2​=3 Its mmm-th term is Sm=3+(m−1)3=3mS_m=3+(m-1)3=3mSm​=3+(m−1)3=3m

  2. Find the last term included in each progression

    • Up to the 25th25^{\text{th}}25th term of the first AP: T25=4+24⋅5=124T_{25}=4+24\cdot 5=124T25​=4+24⋅5=124

    • Up to the 37th37^{\text{th}}37th term of the second AP: S37=3⋅37=111S_{37}=3\cdot 37=111S37​=3⋅37=111

    So common terms must be numbers that appear in both APs and are therefore ≤111\le 111≤111.

  3. Condition for a common term

    A common term must satisfy: x≡4(mod5)x\equiv 4\pmod{5}x≡4(mod5) and also x≡0(mod3)x\equiv 0\pmod{3}x≡0(mod3)

    Let x=3kx=3kx=3k. Then: 3k≡4(mod5)3k\equiv 4\pmod{5}3k≡4(mod5) Since 3−1≡2(mod5)3^{-1}\equiv 2\pmod{5}3−1≡2(mod5), k≡8≡3(mod5)k\equiv 8\equiv 3\pmod{5}k≡8≡3(mod5) Hence: k=5t+3k=5t+3k=5t+3 Therefore, x=3(5t+3)=15t+9x=3(5t+3)=15t+9x=3(5t+3)=15t+9

    So the common terms form the sequence: 9,24,39,54,69,84,99,114,…9,24,39,54,69,84,99,114,\ldots9,24,39,54,69,84,99,114,…

  4. Restrict to the given ranges

    We need common terms that are present in both lists.

    Since the second list goes only up to 111111111, we take common terms ≤111\le 111≤111: 9,24,39,54,69,84,999,24,39,54,69,84,999,24,39,54,69,84,99

    The next one is 114>111114>111114>111, so it is not included.

    Thus, the number of common terms is: 777

  5. Check with options

    Option DDD is 777.

Final Answer: 7\boxed{7}7​

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