Question

Find the third proportional to 36 and 18

Answer 1

let the third proportion be x

36 , 18 ,x are in proportion.

therefore
38x = 18^2
36x = 18 x 18
(2 x 18 )x = 18 x 18
x = 9

Answer 2

Let's think the third proportion is x

We know,

a/b = b/c

or, b^2 = ac

Now,

18^2 =36x

or, x = 18×18/36

or, x =9

The third proportion is 9

Hope it helps.