<em>Решение:
</em>
<em>{х²+xy=9+3y;</em>
<em>{3x+2y=-1;</em>
<em>{х²+xy-3y-9=0;</em>
<em>{2y=-3x-1;</em>
<em>{х²+x(-1,5x-0,5)-3(-1,5x-0,5)-9=0</em>
<em>{y=-1,5x-0,5</em>
<em>{х²-1,5х²-0,5x+4,5x+1,5-9=0</em>
<em>{y=-1,5x-0,5</em>
<em>{-0,5х²+4x-7,5=0</em>
<em>{y=-1,5x-0,5</em>
<em> </em>
<em>{х²-8x+15=0</em>
<em>{y=-1,5x-0,5</em>
<em> </em>
<em>х²-8x+15=0</em>
<em>D=64-60=2²</em>
<em>х₁=3</em>
<em>х₂=5</em>
<em>у₁=-1,5*3-0,5=-5</em>
<em>у₂=-1,5*5-0,5=-8</em>
<em>Ответ: x₁=3, у₁=-5; x₂=5, у₂=-8.</em>
1)a18=a1+(18-1)d
a18=70+51
a18=121
2) s40=99
геометрическую прогрессию еще не проходил
А³+12а²+47а+60=а³+12а²+48а+64-а-4=(а+4)³-(а+4)=(а+4)((а+4)²-1)=(а+4)(а+4-1)(а+4+1)=(а+4)(а+3)(а+5)
![\frac{(n+1)!}{n!} = \frac{(n+1)*n!}{n!} =n+1 ](https://tex.z-dn.net/?f=+%5Cfrac%7B%28n%2B1%29%21%7D%7Bn%21%7D+%3D+%5Cfrac%7B%28n%2B1%29%2An%21%7D%7Bn%21%7D+%3Dn%2B1%0A%0A%0A%0A%0A%0A%0A+)
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![\frac{(n+1)!(n+3)}{(n+4)!}=\frac{1}{(n+2)(n+4)}](https://tex.z-dn.net/?f=++%5Cfrac%7B%28n%2B1%29%21%28n%2B3%29%7D%7B%28n%2B4%29%21%7D%3D%5Cfrac%7B1%7D%7B%28n%2B2%29%28n%2B4%29%7D+)