What do athletes misuse to increase muscle strength?

Two circular coils can be arranged in any of the three situations shown in the figure. Their mutual inductance will be:

When electric current is passed through acidified water for $1930 \ s$,$1120 \ mL$ of $H_2$ gas is collected (at $STP$) at the cathode. What is the current passed in amperes?

The angle between the lines whose direction cosines are given by the equations $l^2+m^2-n^2=0$ and $l+m+n=0$ is

Let $P(3, 2, 6)$ be a point in space and $Q$ be a point on the line $\vec{r} = (\hat{i} - \hat{j} + 2\hat{k}) + \mu(-3\hat{i} + \hat{j} + 5\hat{k})$. Then the value of $\mu$ for which the vector $\vec{PQ}$ is parallel to the plane $x - 4y + 3z = 1$ is:

In the given branch $AB$ of a circuit, a current $I = (10t + 5) \, A$ is flowing, where $t$ is time in seconds. At $t = 0$, the potential difference between points $A$ and $B$ $(V_A - V_B)$ is......$V$.