Which of the following compounds is formed by the addition of a mineral acid to an aqueous solution of borax?

$A$ conducting sphere of radius $R$ carrying a charge $q$ is joined to a conducting sphere of radius $2R$ carrying a charge $-2q$. The charge flowing between them will be

$A$ block of metal is heated to a temperature much higher than the room temperature and allowed to cool in a room free from air currents. Which of the following curves correctly represents the rate of cooling?

If the function $y = \sin^{-1} x$, then $(1 - x^2) \frac{d^2 y}{d x^2}$ is equal to

The $K.E.$ and $P.E.$ of a particle executing $SHM$ with amplitude $A$ will be equal when its displacement is-

The angle between the lines $\vec{r}=(2 \hat{i}-3 \hat{j}+\hat{k})+\lambda(\hat{i}+4 \hat{j}+3 \hat{k})$ and $\vec{r}=(\hat{i}-\hat{j}+2 \hat{k})+\mu(\hat{i}+2 \hat{j}-3 \hat{k})$ is