[tex]c)57 {}^{2} - 56 \times 57 = \\ = 57(57 - 56) = 57 \times 1 = 57 \\ \\ f)89 {}^{3} - 88 \times {89}^{2} - 89 \times 90 = \\ = 89 {}^{2} (89 - 88) - 89 \times 90 = \\ = 89 {}^{2} \times 1 - 89 \times 90 = \\ = 89(89 -90) = 89 \times( - 1 )= - 89[/tex]
