An isosceles triangle has congruent sides of 20 cm. the base is 10 cm. find the length of the triangle.

If we draw  a line from the vertex to the midpoint of the base we get 2 right angled triangles. So we can use Pythagoras theorem to find the length ( height):-

20^2 = x^ + 5^2
x^2 = 400 - 25 = 375
x =   sqrt 375  =  19.36 cm   to nearest hundredth.