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°, what is $A^{p}rime$ ?

Solution: Here, U is the set of all triangles.
A is the set of all triangles with at least one angle different from ${60}^{\circ }$.
So, there can be one angle, two angles or three angles that can not be ${60}^{\circ }$
Thus, $A^{\prime }$ = $U-A$
So,$A^{\prime }$ will be a set of triangle that contains all angles with ${60}^{\circ }$ that means set of all equilateral triangles.