Crystals of which pair are isomorphous?

Vertebral column is derived from

If the length of a stretched string is shortened by $40\%$ and the tension is increased by $44\%$,then the ratio of the final and initial fundamental frequencies is

The equation of the circle with radius $5$ and touching the coordinate axes in the third quadrant is

The two surfaces of a biconvex lens have the same radii of curvature. This lens is made of glass with a refractive index of $1.5$ and has a focal length of $10 ~cm$ in air. The lens is cut into two equal halves along a plane perpendicular to its principal axis to yield two plano-convex lenses. The two pieces are glued such that the convex surfaces touch each other. If this combination lens is immersed in water (refractive index $=$ $4/3$),its focal length (in $cm$) is:

An inductor of inductance $L=400\,mH$ and resistors of resistance $R_1=2\,\Omega$ and $R_2=2\,\Omega$ are connected to a battery of emf $E=12\,V$ as shown in the figure. The internal resistance of the battery is negligible. The switch $S$ is closed at $t=0$. The potential drop across $L$ as a function of time is