Let $U$ be the set of all triangles in a plane. If $A$ is the set of all triangles with at least one angle different from $60^{circ}$, what is $A^{prime}$ ?
Question:
Let $U$ be the set of all triangles in a plane. If $A$ is the set of all triangles with at least one angle different from $60^{\circ}$, what is $A^{\prime}$ ?
Solution:
$\mathrm{A}^{\prime}$ is the set of all equilateral triangles.