$\frac{4.2 \times 4.2 - 1.9 \times 1.9}{2.3 \times 6.1} = ?$

What will come in the place of the question mark $(?)$ in the following expression?
$3 \frac{3}{4} + 4 \frac{2}{5} - 3 \frac{1}{8} = ?$

If $(x + \frac{1}{x})^2 = 3$,then $(x^3 + \frac{1}{x^3})$ is equal to

If $a, b, c$ are three non-zero real numbers such that $a+b+c=0$ and $b^{2} \neq ca,$ then the value of $\frac{a^{2}+b^{2}+c^{2}}{b^{2}-ca}$ is

What is the simplified value of $(2+1)(2^{2}+1)(2^{4}+1)(2^{8}+1)?$

Find the value of the following expression up to four places of decimals:
$\left[1+\frac{1}{1 \times 2}+\frac{1}{1 \times 2 \times 4}+\frac{1}{1 \times 2 \times 4 \times 8}+\frac{1}{1 \times 2 \times 4 \times 8 \times 16}\right]$