Which of the following complex ions is not expected to absorb visible light?

$H_3BO_3$ is

$A$ convex mirror of focal length $f$ forms an image which is $1/n$ times the object. The distance of the object from the mirror is

$A$ coil of radius '$r$' is placed on another coil (whose radius is '$R$' and current through it is changing) so that their centers coincide. $(R \gg r)$. If both are coplanar,then the mutual inductance between them is proportional to

If the difference between the roots of the equations ${x^2} + ax + b = 0$ and ${x^2} + bx + a = 0$ is the same,and $a \ne b$,then:

Let $a, b \in R$. If the mirror image of the point $P(a, 6, 9)$ with respect to the line $\frac{x-3}{7} = \frac{y-2}{5} = \frac{z-1}{-9}$ is $(20, b, -a-9)$,then $|a+b|$ is equal to: