[tex]a) {2009}^{2} - 2009 \times 2010 \\ = 2009(2009 - 2010) \\ = 2009 \times ( - 1) \\ = - 2009[/tex]
[tex]b) {2009}^{2} - 2008 \times 2009 \\ = 2009(2009 - 2008) \\ = 2009 \times 1 \\ = 2009[/tex]
[tex]c)5 + 10 + 15 + ... + 2005 - 5 \times (1 + 2 + 3 + ... + 401) \\ = 5(1 + 2 + 3 + ... + 401) - 5(1 + 2 + 3 + ... + 401) \\ = 0[/tex]
