(yi-ybar)^2=yi^2+ybar^2-2*ybar*yi

use (c)

s^2=1/(n-1)*[Σyi^2+Σybar^2-Σ2*ybar*yi]

use (a)

s^2=1/(n-1)*[Σyi^2+n*ybar^2-Σ2*ybar*yi]

use (b)

s^2=1/(n-1)*[Σyi^2+n*ybar^2-2*ybar*Σyi]

since ybar=Σyi/n

s^2=1/(n-1)*[Σyi^2+(Σyi)^2/n-2*(Σyi)^2/n]

=1/(n-1)*[Σyi^2-(Σyi)^2/n]

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好长时间没做数学题了。。。