If 2 n = 128, find the value of (2 n - 1 )(5 n
MATHEMATICS
WAEC 2008
If 2n = 128, find the value of (2n - 1)(5n - 1)
- A. 5(106)
- B. 2(106)
- C. 5(105)
- D. 2(105)
Correct Answer: D. 2(105)
Explanation
2n = 128
2n = 27
n = 7
(2n - 1)(5n - 2) = (2n - 2.2)(5n - 2) put n = 7
(2n - 1)(5n - 2) = 2(2n - 2 x 5n - 2)
= 2(2 x 5)n - 2
= 2(10n - 2) put n = 7
(2n-1)(5n-2) = 2(105)
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