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                  <h1>Ex. 3.9</h1>
                
                <details class="question" open="open"><summary>Ex. 3.9</summary><p><em>Forward stepwise regression</em>.  Suppose we have the QR decomposition for the <span class="arithmatex">\(N\times q\)</span> matrix <span class="arithmatex">\(\textbf{X}_1\)</span> in a multiple regression problem with response <s