Now, as you know the product of roots is 30, let those roots be a, b, c ϵR. The minimum value of 10a + 9b + 10c is
Let us take 3 terms 10a, 9b, 10c
We know that AM≥ GM
10a+9b+10c3 ≥ (10×9×10×abc)13
10a + 9b + 10c ≥3× (10×10×9×30)13
10a + 9b + 10c ≥ 90
So, the minimum value is 90.