Condition for Two Circles to Touch

Circles $$C_1$$ (center $$(h_1,k_1)$$, radius $$r_1$$) and $$C_2$$ (center $$(h_2,k_2)$$, radius $$r_2$$):
- External touch: $$d = |r_1 + r_2|$$
- Internal touch: $$d = |r_1 - r_2|$$
where $$d = \sqrt{(h_1-h_2)^2 + (k_1-k_2)^2}$$