Let , ; > , be the roots of the equation x2 - 2x - 3 = 0. Let Pn = n - n, n N. Then (11 3 - 10 2)P10 + (11 2 + 10)P11 - 11P12 is equal to
- 10 3P9
- 11 3P9
- 11 2P9
- 10 2P9
Answer
(1) 10 3P9
Solution
Since 2 = 2 + 3 and 2 = 2 + 3, multiplying by n, n and subtracting gives
Pn + 2 = 2Pn + 1 + 3Pn
Putting 11P12 = 11 2P11 + 11 3P10, the 11 3P10 and 11 2P11 terms cancel and the expression becomes
10P11 - 10 2P10 = 10(P11 - 2P10) = 10 3P9
(using the same relation with n = 9)
Answer: (1)