If they have a common centre, they are concentriccircles.
Circles in the same plane with the same center are concentric circles.
Tangent circles.
concentric circles
concentric circles
Concentric Circle
If they have a common centre, they are concentriccircles.
They're definitely similar, concentric, and coplanar. They're not necessarily congruent, but they could be.
Difficult to tell since there is nothing following. My guess is concentric, but that does not require the circles to be coplanar.
When the centers of both the circles are at the same point.
Circles in the same plane with the same center are concentric circles.
Yes, it is. Circles that are in the same plane and having the same center are called congruent circles.
Tangential circles.
"Coplanar" means "on the same plane". For example, if a laser gun shot straight through the center of both circles, then unless they both have the same center, it wouldn't just pierce the two circles, but would slice them clean in half. In simple terms, the definition of "plane" is "a flat surface". In mathematics, this means a 2-dimensional surface that extends in every compass direction (or at least if the compass were held in a certain position). This is also known as an "affine space of rank 2". In three dimensions, any plane can be described by ax+by+cz=d. Using that along with the equations for both circles, you should be able to solve for a,b,c,d. If you can't, they aren't coplanar.
Tangent circles.
Tangential circles.
concentric circles