Q.What is the value of the MR when the demand curve is elastic?

The sign of marginal revenue is tied directly to elasticity along the demand curve.

There is a fixed link between marginal revenue and the price elasticity of demand. From $MR = AR\left(1 - \dfrac{1}{e}\right)$, whenever $e > 1$ the bracket $\left(1 - \dfrac{1}{e}\right)$ is positive, so $MR$ is positive. Thus, over the elastic range of the demand curve, marginal revenue has a positive value (it becomes zero at unitary elasticity, $e = 1$, and negative in the inelastic range, $e < 1$).