Let A and G be the arithmetic and geometric mean for a, b respectively.
Thn given that, a+b = 6G
(a+b)/2 = 3G
A = 3G
Let take a quadratic equation in x whose roots are a and b. Then, x^2 - (a+b)x + ab = 0
x^2 - 2Ax + G^2 = 0
Using quadratic formula, we get x = [2A +- root(4A^2 - 4G^2)] / 2
x = A +- sqrt(A^2 - G^2)
Since a and b are the roots of the then, a = A + sqrt(A^2 - G^2) & b = A - sqrt(A^2 -G^2)
Then,
Now, putting A = 3G in the above equation we get,



Hence proved.....