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

Here U = {x : x is a triangle} A = {x : x is a triangle and has at least one angle different from ${60}^{\circ }$}

$\therefore A^{\prime }$ = U - A

= {x : x is a triangle} - {x : x is a triangle and has atleast one angle different from ${60}^{\circ }$}

= {x : is a triangle and has all angles equal to ${60}^{\circ }$}

= Set of all equilateral triangles.