Let ABC be the triangle and BC be its base.
Let AD be the line joining point A to base BC.
Given △ABC is isosceles, hence line joining the vertex to midpoint of its base is 90o
Here △ABD and △ADC are right angled triangles and AB & AC
are the hypotenuses of △ABD and △ADC respectively.
Since a hypotenuse is greater than any of the sides of triangle,
AB>AD and also AC>AD.