Multiple choice If the roots of the equation $x^3+ax^2+bx+c=0$ are in A.P., then $2a^3-9ab= $_____. $9c$ $18c$ $27c$ $-27c$ Reveal answer Fill a bubble to check yourself D Correct answer Explanation If roots are a-d, a, a+d, then sum = 3a = -a (coefficient of x^2), so a = -a/3. Substituting into the cubic equation and using Vieta's formulas leads to the identity 2a^3 - 9ab = -27c.