Why does K+ going out of the cell cause hyperpolarization?
Adding K+ on the outside of membrane is not the same as transfering K+ from inside to outside.
When K+ is added to outside(hyperkalemia) the RMP becomes less negative(depolarisation)
When K+ is transfered from in to out RMP becomes more negative(re/hyperpolarisation)
We should not forget that most of the intracellular anions are nonpermeable protiens, so when K+ is transfered to outside they pull harder, and when K+ added outside, the pull decreases