Short Answer TypeFactorise the following expression:
q2 - 10q + 21
We have  q2 - 10q + 21 = q2 - 10q + 25 - 4   [∵  21 = 25 - 4]
                               Â
                                = [(q)2 - 2(q)(5) + (5)2] - (2)2
                                          Â
                                          = [q - 5]2 - [2]2
                                     = [(q - 5) + 2][(q - 5) - 2]
                                                   Â
                                                               [Using a2 - b2 = (a + b) (a - b)]
                                = (q - 3) (q - 7)
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