At the base of an isosceles triangle, the angle is 30 degrees. Find the angle between the sides.

In order to find the angle between the lateral sides of an isosceles triangle, we will first of all recall the properties of the angles at the base of an isosceles triangle and the theorem on the sum of the angles of a triangle.

So, the angles at the base of an isosceles triangle are equal to each other.

And the sum of the angles of a triangle, according to the theorem on the sum of the angles of a triangle, is 180 °.

Let’s denote by x the degree measure of the angle at the apex of an isosceles triangle.

We get the equation:

30 + 30 + x = 180;

x = 180 – 60;

x = 120 °.

Answer: 120 °.



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