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 $A^{\prime }?$

Answer 1

$U=$ Set of all triangles of a plane.
$A=$ Set of all triangles with at least one angle different from ${60}^{\circ }$
$\therefore A^{\prime }$ = Set of all triangles with no angle different from ${60}^{\circ }$
$\Rightarrow A^{\prime }$ = Set of all trinagles with all three angles ${60}^{\circ }$
$\Rightarrow A^{\prime }$ = Set of all equilateral triangles.