Which of the following is the correct order of lattice energy for the given ionic compounds?

The circle $4x^2 + 4y^2 - 12x - 12y + 9 = 0$

Two sources $A$ and $B$ are sending notes of frequency $680 \, Hz$. A listener moves from $A$ towards $B$ with a constant velocity $u$. If the speed of sound in air is $340 \, ms^{-1}$, what must be the value of $u$ so that the listener hears $10$ beats per second (in $ \, ms^{-1}$)?

In a compound microscope,the focal length of the objective lens is $f_o = 1.2 \ cm$ and the focal length of the eyepiece is $f_e = 3.0 \ cm$. When the object is kept at $u_o = -1.25 \ cm$ in front of the objective,the final image is formed at infinity. The magnifying power of the compound microscope is:

Which of the following fluorides does not exist?

Which metal is protected by a layer of its own oxide?