For an integer n 2, if the arithmetic mean of all coefficients in the binomial expansion of (x + y)2n - 3 is 16, then the distance of the point P (2n - 1, n2 - 4n ) from the line x + y = 8 is
- 2
- 2 2
- 5 2
- 3 2
Answer
(4) 3 2
Solution
In (x + y)2n - 3 there are 2n - 2 coefficients and their sum is 22n - 3 (put x = y = 1).
Arithmetic mean = 22n - 32n - 2 = 16
22n - 3 = 32(n - 1)
Trying integers n 2: n = 5 gives 27 = 128 = 32 4 (the left side grows much faster, so n = 5 is the only solution).
P(2n - 1, n2 - 4n) = P(9, 5)
Distance of P from x + y - 8 = 0:
d = |9 + 5 - 8| 12 + 12 = 6 2 = 3 2
Answer: (4)