# 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′?

**Solution:**

It is given that U is the set of all triangles in a plane.

Also, A is the set of all triangles with at least one angle different from 60°.

Then A', which is the complement of A, is

A' = U - A

= {All triangles} - {all triangles with at least one angle different from 60°}

= {All triangles in which all angles are equal to 60°}

= {All equilateral triangles}

**Thus, A′ is the set of all equilateral triangles**

NCERT Solutions Class 11 Maths Chapter 1 Exercise 1.5 Question 6

## 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′?

**Summary:**

If U is the set of all triangles in a plane and A is the set of all triangles with at least one angle different from 60°, then A′ is the set of all equilateral triangles