Angle between Two Planes

Q. What is the angle between two planes having normal vectors as
$\overrightarrow{n_1}=\hat{i}+2\hat{j}+2\hat{k}$ and $\overrightarrow{n_2}=4\hat{i}-4\hat{j}+2\hat{k}$ ?

1. 
   $0^o$
2. 
   ${90}^{\circ }$
3. 
   ${45}^{\circ }$
4. 
   ${60}^{\circ }$

Q. Angle between 2 planes $\overline{r}.\widehat{n_1}=d_1,\overline{r}.\widehat{n_2}=d_2$, can always be given by

1. $\theta =si{n}^{-1}|\widehat{n_1}\times \widehat{n_2}|$
2. $\theta =co{s}^{-1}(\widehat{n_1}.\widehat{n_2})$
3. $\theta =si{n}^{-1}|\widehat{n_1}.\widehat{n_2}|$
4. $\theta =co{s}^{-1}|(\widehat{n_1}.\widehat{n_2})|$

Q. Solve the following equations:
$x{y}^{\frac{1}{2}}+y{x}^{\frac{1}{2}}=20$,
$x^{\frac{3}{2}}+y^{\frac{3}{2}}=65$.

1. $(1,16)$
2. $(16,1)$
3. $(3,15)$
4. $(2,10)$