If P(11, r) = P (12, r – 1), find n.
Given: P(11, r) = P (12, r – 1)
To find: value of n
We know,
So, according to question:
P (11, r) = P (12, r – 1)
⇒ (13 – r) (12 – r) = 12
⇒ 156 – 12r – 13r + r2 = 12
⇒ 156 – 12 – 25r + r2 = 0
⇒ r2 – 25r + 144 = 0
⇒ r2 – 16r – 9r + 144 = 0
⇒ r(r – 16) – 9(r – 16) = 0
⇒ (r – 9) (r – 16) = 0
⇒ r = 9 or 16
For, P(n, r): r ≤ n
∴ r = 9 {for, P(11, r)}