In an isosceles triangle ABC with base AC, angle B = 40 degrees. Find AC.

In order to find the values ​​of the angles A and C at the base AC in an isosceles ΔABC, given that the angle B is 40 °, you first need to recall the rule.
According to this rule, in an isosceles triangle, the angles at the base are equal.
In the considered ΔABC, the base is AC.
Thus, the angles of unknown magnitude, as well as the angles at the base AC, are angles A and C.
Now let’s write for angles A and C.
A = C.
Also, remember the rule that the sum of all the angles of any triangle is 180 °.
Now we write down ΔABC for angles and calculate the degree measure of angles A and C.
A + B + C = 180 °.
2A + B = 180 °.
2A = 180 ° – B.
2A = 180 ° – 40 °.
2A = 140 °.
A = 140 °: 2.
A = 70 °.
A = C = 70 °.
Answer: 70 °; 70 °.