Question

the vertical angle of an isosceles triangle is 100 degree find its base angle

Answer 1

In isosceles traingle Two sides are equal hence angles other than vertical angle will be same (equal sides has equal angles opposite it)
let one Angle be x and other be also x
then
acc to angle sum property of traingle
2x+100=180
2x=80
x=40. base angle =40

Answer 2

Here is your answer

let the base angle is x
so. x +x +100 =180
2x =180- 100
2x =80
x =80/2
x =40
Hence base angle are 40

Hope it helps