Product of any r consecutive natural numbers is always divisible by
r!
(r + 4)!
(r + 1)!
(r + 2)!
If C0, C1, C2, ..., Cn denote the coefficients in the expansion of (1 + x)n, then the value of C1 + 2C2 + 3C3 + ... + nCn is
n . 2n - 1
(n + 1)2n - 1
(n + 1)2n
(n + 2)2n - 1
A.
n . 2n - 1
Since,
(1 + x)n = C0 + x . C1 + x2 . C2 + ... + xn . Cn
On differentiating both sides w.r.t. x, we get
n(1 + x)n - 1 = C1 + 2xC2 + ... + nxn - 1Cn
Put x = 1, we get
n(2)n - 1 = C1 + 2. C2 + 3 . C3 + ... n . Cn
The angle between the lines joining the foci of an ellipse to one particular extremity of the minor axis is 90°. The eccentricity of the ellipse is
1/8
1/√3
√(2/3)
√(1/2)