" Which of the following may be positive or negative? "

Which of the following enthalpy may be positive or negative?

If r is negative, which of the following must also be negative?

(a) The linear magnification of a concave lens is always positive. Why? (b) The linear magnification of a convex lens may be positive or negative. Why ?

Which of the following is an example of a positive ion and negative ion that is isoelectronic with Argon