Multiple choice The mean deviation from mean of the observation $a,\ a+d,\ a+2d,....,a+2nd$ is $\dfrac {n(n+1)d^{2}}{3}$ $\dfrac {n(n+1)}{2}d^{2}$ $a+\dfrac {n(n+1)d^{2}}{2}$ $None\ of\ these$ Reveal answer Fill a bubble to check yourself A Correct answer