Hi,

I know it might sound stupid but I do not understand why the fact that sum of residuals equals zero implies the sum of (residual i*Xi) equals the sum of (residual i *(Xi-average of Xi). The equation in the image attached(where u means residual) seems to be based on the same logic which I do not understand. Also, why the sum of (residual i*Yi) equals zero?

Thank you very much if you can shed some light onto this.

I know it might sound stupid but I do not understand why the fact that sum of residuals equals zero implies the sum of (residual i*Xi) equals the sum of (residual i *(Xi-average of Xi). The equation in the image attached(where u means residual) seems to be based on the same logic which I do not understand. Also, why the sum of (residual i*Yi) equals zero?

Thank you very much if you can shed some light onto this.

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