passpadi

2007

Mathematics

JAMB

The nth term of the sequence 3/2, 3, 7, 16, 35, 74 ..... Is

A.

5 . 2n-2 - n

B.

5 . 2n-2 - (n+1) / 2

C.

3 . 2n-2

D.

3/2 n

Correct Answer: 5 . 2n-2 - n

Explanation

3/2, 3, 7, 16, 35, 74, .... Using the method of substitution When n = 1, 5 . 2n-2 - 1 = 5 . 21-2 - 1 = 5 x 2-1 - 1 = 5 x 1/2 - 1 = 5/2 - 1 = 3/2 When n = 1, 5 . 2n-2 - n = 5 . 22-2 - 2 = 5 x 20 - 2 = 5 x 1 – 2 = 3 When n = 3, 5 . 2n-2 - n = 5 . 23-2 - 3 = 5 x 21 - 3 = 5 x 2 – 3 = 10 -3 = 7 When n = 4, 5 . 2n-2 - n = 5 . 24-2 - 4 = 5 x 22 - 4 = 5 x 4 – 4 = 20 – 4 = 16 When n = 5, 5 . 2n-2 - n = 5 . 25-2 - 5 = 5 x 23 - 5 = 5 x 8 – 5 = 40 – 5 = 35 When n = 6, 5 . 2n-2 - n = 5 . 26-2 - 6 = 5 x 24 - 6 = 5 x 16 – 6 = 80 – 6 = 74 ∴ 5 . 2n-2 - 1 = 3/2, 3, 7, 16, 35, 74

3/2, 3, 7, 16, 35, 74, .... Using the method of substitution When n = 1, 5 . 2n-2 - 1 = 5 . 21-2 - 1 = 5 x 2-1 - 1 = 5 x 1/2 - 1 = 5/2 - 1 = 3/2 When n = 1, 5 . 2n-2 - n = 5 . 22-2 - 2 = 5 x 20 - 2 = 5 x 1 – 2 = 3 When n = 3, 5 . 2n-2 - n = 5 . 23-2 - 3 = 5 x 21 - 3 = 5 x 2 – 3 = 10 -3 = 7 When n = 4, 5 . 2n-2 - n = 5 . 24-2 - 4 = 5 x 22 - 4 = 5 x 4 – 4 = 20 – 4 = 16 When n = 5, 5 . 2n-2 - n = 5 . 25-2 - 5 = 5 x 23 - 5 = 5 x 8 – 5 = 40 – 5 = 35 When n = 6, 5 . 2n-2 - n = 5 . 26-2 - 6 = 5 x 24 - 6 = 5 x 16 – 6 = 80 – 6 = 74 ∴ 5 . 2n-2 - 1 = 3/2, 3, 7, 16, 35, 74
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