Упростите выражение: 1/(a+7)+10/(21-4a-a^2).

\[\frac{1}{a + 7} + \frac{10}{21 - 4a - a^{2}} =\]

\[= \frac{1^{\backslash a - 3}}{a + 7} + \frac{10}{(a - 3)(a + 7)} =\]

\[= \frac{a - 3 + 10}{(a - 3)(a + 7)} =\]

\[= \frac{a + 7}{(a - 3)(a + 7)} = \frac{1}{a - 3}\]

\[a^{2} + 4a - 21 = (a + 7)(a - 3)\]

\[D_{1} = 4 + 21 = 25\]

\[a_{1} = - 2 + 5 = 3;\]

\[a_{2} = - 2 - 5 = - 7.\]