Question 1121817
x bar=X.

3.SUM[(xi-a)^2]=SUM[xi^2]+na^2-2a*(SUM[xi])=SUM[xi^2]+na^2-2anX
1.SUM[(xi-X)^2]=SUM[xi^2]+nX^2-2X*SUM[xi]=SUM[xi^2]+nX^2-2nX^2=SUM[xi^2]-nX^2
2.n(X-a)^2=nX^2+na^2-2anX.

1+2=3. Done.