If the coefficients of x² and x³ in the expansion of (3 + ax)⁹ be same, then the value of a is

using binomial theorem, the value of (0.999)³ correct to 3 decimal places is

$$({2^{3n}} - 1)$$ will be divisible by $$(\forall n \in N)$$

If in the expansion (a $$-$$ 2b)ⁿ, the sum of the 5^th and 6^th term is zero, then the value of $${a \over b}$$ is