A sequence is given by \(2\frac{1}{2}, 5, 7\frac{1}{2}, .....\) if the nth term is 25, find n

  • A 9
  • B 10
  • C 12
  • D 15
waec 2005mathematics

The correct answer is B. 10

\(a = 2\frac{1}{2}, nth = a + (n-1)d \Rightarrow 25 = 2\frac{1}{2} + (n-1)2\frac{1}{2}\\

25 = \frac{5}{2}+(n-1)\frac{5}{2} \Rightarrow 22\frac{1}{2} = \frac{5n-5}{2}\Rightarrow \frac{45}{2} = \frac{5n-5}{2}\\

45 = 5n - 5 \Rightarrow 5n = 50 \Rightarrow n = 10\)

Previous question Next question