Thank you for reporting, we will resolve it shortly
Q.
The coefficient of $c ^2 d ^2$ in the expansion of $(\sqrt{ c }+\sqrt{ d })^8$, is
Binomial Theorem
Solution:
$(\sqrt{ c }+\sqrt{ d })^8 ; \quad{ }^8 C _{ r } c ^{\frac{8- r }{2}} d ^{\frac{ r }{2}} \Rightarrow r =4$
$\text { coefficient } ={ }^8 C _4=\frac{8 \times 7 \times 6 \times 5}{24}=70 . $