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a₄=-9
a₇=-21
![d= frac{a_7-a_4}{7-4}= frac{-21-(-9)}{3}= frac{-12}{3}=-4 d= frac{a_7-a_4}{7-4}= frac{-21-(-9)}{3}= frac{-12}{3}=-4](https://tex.z-dn.net/?f=d%3D+frac%7Ba_7-a_4%7D%7B7-4%7D%3D+frac%7B-21-%28-9%29%7D%7B3%7D%3D+frac%7B-12%7D%7B3%7D%3D-4)
a₁=a₄-3d=-9-3•(-4)=-9+12=3
n∈N
![S_n= frac{a_1+a_n}{2}*n= frac{a_1+a_1+d(n-1)}{2}*n= frac{2a_1+d(n-1)}{2}*n\S_n= frac{2*3-4(n-1)}{2}*n=-150\S_n= frac{2(3-2(n-1))}{2}*n=-150\S_n=(3-2(n-1))*n=-150\S_n=(3-2n+2)*n=-150\(5-2n)*n=-150\5n-2n^2+150=0\-2n^2+5n+150=0\2n^2-5n-150=0\D= b^2-4ac=25+8*150=1225\n_1= frac{-b+ sqrt{D}}{2a}= frac{5+ sqrt{1225}}{2*2}= frac{5+35}{4}= frac{40}{4}=10\n_2=ne//udovletvorayet\OTBET:n=10. S_n= frac{a_1+a_n}{2}*n= frac{a_1+a_1+d(n-1)}{2}*n= frac{2a_1+d(n-1)}{2}*n\S_n= frac{2*3-4(n-1)}{2}*n=-150\S_n= frac{2(3-2(n-1))}{2}*n=-150\S_n=(3-2(n-1))*n=-150\S_n=(3-2n+2)*n=-150\(5-2n)*n=-150\5n-2n^2+150=0\-2n^2+5n+150=0\2n^2-5n-150=0\D= b^2-4ac=25+8*150=1225\n_1= frac{-b+ sqrt{D}}{2a}= frac{5+ sqrt{1225}}{2*2}= frac{5+35}{4}= frac{40}{4}=10\n_2=ne//udovletvorayet\OTBET:n=10.](https://tex.z-dn.net/?f=S_n%3D+frac%7Ba_1%2Ba_n%7D%7B2%7D%2An%3D+frac%7Ba_1%2Ba_1%2Bd%28n-1%29%7D%7B2%7D%2An%3D+frac%7B2a_1%2Bd%28n-1%29%7D%7B2%7D%2An%5CS_n%3D+frac%7B2%2A3-4%28n-1%29%7D%7B2%7D%2An%3D-150%5CS_n%3D+frac%7B2%283-2%28n-1%29%29%7D%7B2%7D%2An%3D-150%5CS_n%3D%283-2%28n-1%29%29%2An%3D-150%5CS_n%3D%283-2n%2B2%29%2An%3D-150%5C%285-2n%29%2An%3D-150%5C5n-2n%5E2%2B150%3D0%5C-2n%5E2%2B5n%2B150%3D0%5C2n%5E2-5n-150%3D0%5CD%3D+b%5E2-4ac%3D25%2B8%2A150%3D1225%5Cn_1%3D+frac%7B-b%2B+sqrt%7BD%7D%7D%7B2a%7D%3D+frac%7B5%2B+sqrt%7B1225%7D%7D%7B2%2A2%7D%3D+frac%7B5%2B35%7D%7B4%7D%3D+frac%7B40%7D%7B4%7D%3D10%5Cn_2%3Dne%2F%2Fudovletvorayet%5COTBET%3An%3D10.)
УДАЧИ ВАМ ВО ВСЁМ)))!
a₇=-21
a₁=a₄-3d=-9-3•(-4)=-9+12=3
n∈N
УДАЧИ ВАМ ВО ВСЁМ)))!
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