Sequence recurrence relation solution

CAT 2005 Slot 1 · QA · Hard · Algebra

If a1=1a_1 = 1 and an+13an+2=4na_{n+1} - 3a_n + 2 = 4n for every positive integer nn, then a100a_{100} equals

  1. A.

    3992003^{99} - 200

  2. B.

    399+2003^{99} + 200

  3. C.

    31002003^{100} - 200

  4. D.

    3100+2003^{100} + 200

Answer

C

Explanation

Recurrence: an+1=3an+4n2a_{n+1} = 3a_n + 4n - 2. Given a1=1a_1 = 1. a2=3(1)+4(1)2=5a_2 = 3(1) + 4(1) - 2 = 5. a3=3(5)+4(2)2=21a_3 = 3(5) + 4(2) - 2 = 21. Testing option C (3n2n3^{n} - 2n or 3n2003^n - 200 for n=100n=100): For n=1n=1: 312(1)=1=a13^1 - 2(1) = 1 = a_1. For n=2n=2: 322(2)=5=a23^2 - 2(2) = 5 = a_2. For n=3n=3: 332(3)=21=a33^3 - 2(3) = 21 = a_3. In general, an=3n2na_n = 3^n - 2n. Therefore, a100=3100200a_{100} = 3^{100} - 200.

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