Water is oxidised to oxygen by

The common Indian bullfrog is:

$\sum_{k=1}^{\infty} \frac{1}{k!}\left(\sum_{n=1}^k 2^{n-1}\right)$ is equal to

$A$ body,thrown upwards with some velocity,reaches a maximum height of $50 \ m$. Another body with double the mass thrown up with double the initial velocity will reach a maximum height of $.......... \ m$.

If $A, B, C, D$ are angles of a cyclic quadrilateral,then $\cos A + \cos B + \cos C + \cos D$ is equal to

$\frac{1}{2 \cdot 3} + \frac{1}{4 \cdot 5} + \frac{1}{6 \cdot 7} + \frac{1}{8 \cdot 9} + \dots$ is equal to