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6.
d=8-10=-2
Sn=(2а1+d(n-1))*n/2
S7=(2*10-2(7-1))*7/2=(20-12)*7/2=28.

7.
$\frac{m}{m + n} + \frac{n}{m - n} = \\ = \frac{m(m - n) + n(m + n)}{(m + n)(m - n)} = \\ = \frac{ {m}^{2} - mn + mn + {n}^{2} }{ {m}^{2} - {n}^{2} } = \\ = \frac{ {m}^{2} + {n}^{2} }{ {m}^{2} - {n}^{2} } = \\ = \frac{ { \sqrt{10} }^{2} + { \sqrt{5} }^{2} }{ { \sqrt{10} }^{2} - { \sqrt{5} }^{2} } = \\ = \frac{10 + 5}{10 - 5} = \\ = \frac{15}{5} = 3$
8. 5х-2(х-1)≥17+4х
5х-2х+2≥17+4х
3х-4х≥17-2
-х≥15
х≤-15
хє(-∞;-15]