The distance between the centres of the two circles of radii r1, and r2 is d. They will touch each other internally if
The distance between centres of two circles = d
Radius of both circles = $$r_1$$ and $$r_2$$
For the circles to touch each other internally,
Here, AC = $$r_1$$ and BC = $$r_2$$
Distance between their centres = AB = d
Now, clearly AB = AC - BC
=> d = $$r_1-r_2$$
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