# Linear regression problem: how do I prove that $\sum_{i=1}^{n}(Y_{i} - \hat{Y}_{i}) = 0$? [closed]

If $$\textbf{X}\in\textbf{R}^{n\times p}$$ has full rank and $$\textbf{Y}\in\textbf{R}^{n\times 1}$$, prove that \begin{align*} \sum_{i=1}^{n}(Y_{i} - \hat{Y}_{i}) = 0 \end{align*}

where $$\hat{\textbf{Y}}$$ is the fitted value of the linear model $$\textbf{Y} = \textbf{X}\beta + \textbf{e}$$, $$\textbf{e}\sim\mathcal{N}_{n}(\textbf{0},\sigma^{2}\textbf{I}_{n})$$. Precisely speaking, $$\hat{\textbf{Y}} = \textbf{X}\hat{\beta}$$ such that $$\textbf{X}^{\prime}\textbf{X}\hat{\beta} = \textbf{X}^{\prime}\textbf{Y}$$. Can someone help me? Thanks!

## closed as off-topic by Michael Chernick, kjetil b halvorsen, Peter Flom♦Mar 31 at 12:46

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• Hint: the first column of $\mathbf{X}$ is all $1$'s – Francis Mar 31 at 4:11
• Sorry, I still did not grasp the idea. Can you provide a full answer? – user1337 Mar 31 at 4:40
• Possible duplicate of Do these residuals sum to zero? – kjetil b halvorsen Mar 31 at 12:43
