Mathematics

The 21st term of an A.P. whose first two terms are -3 and 4 is

  1. 17

  2. 137

  3. 143

  4. -143

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Answer

Given a = -3 and a2 = 4

We know that

    an = a + (n - 1)d
∴ a2 = -3 + (2 - 1) × d
⇒ 4 = -3 + d
⇒ d = 4 + 3
⇒ d = 7.

    a21 = -3 + (21 - 1) × 7
⇒ a21 = -3 + 20 × 7
⇒ a21 = -3 + 140
⇒ a21 = 137.

Hence, Option 2 is the correct option.

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