7х-12=3(9х+8)-2х
7х-12=27х+24-2х
7х-27х+2х=24+12
-18х=36
х=36 / (-18)
х= -2
![S_n= \frac{a_1+a_n}{2} *n= \frac{a_1+a_1+(n-1)d}{2} *n= \frac{2a_1+(n-1)d}{2} *n](https://tex.z-dn.net/?f=S_n%3D+%5Cfrac%7Ba_1%2Ba_n%7D%7B2%7D+%2An%3D+%5Cfrac%7Ba_1%2Ba_1%2B%28n-1%29d%7D%7B2%7D+%2An%3D+%5Cfrac%7B2a_1%2B%28n-1%29d%7D%7B2%7D+%2An)
- сумма n членов арифметической прогрессии
n=20
a)
![a_1=7](https://tex.z-dn.net/?f=a_1%3D7)
d=2,1
![S_{20}= \frac{2*7+19d}{2} *20=(14+19*2,1)*10=(14+39,9)*10=539](https://tex.z-dn.net/?f=S_%7B20%7D%3D+%5Cfrac%7B2%2A7%2B19d%7D%7B2%7D+%2A20%3D%2814%2B19%2A2%2C1%29%2A10%3D%2814%2B39%2C9%29%2A10%3D539)
б)
![a_1=-3](https://tex.z-dn.net/?f=a_1%3D-3)
d=0,2
![S_{20}=(2*(-3)+19*0,2)*10=(-6+3,8)*10=-22](https://tex.z-dn.net/?f=S_%7B20%7D%3D%282%2A%28-3%29%2B19%2A0%2C2%29%2A10%3D%28-6%2B3%2C8%29%2A10%3D-22)
в)
![a_1=17](https://tex.z-dn.net/?f=a_1%3D17)
d=-1,3
![S_{20}=(2*17+19*(-1,3))*10=(34-24,7)*10=93](https://tex.z-dn.net/?f=S_%7B20%7D%3D%282%2A17%2B19%2A%28-1%2C3%29%29%2A10%3D%2834-24%2C7%29%2A10%3D93)
г)
![a_1=-19](https://tex.z-dn.net/?f=a_1%3D-19)
d=2
![S_{20}=(2*(-19)+19*2)*10=(-38+38)*10=0](https://tex.z-dn.net/?f=S_%7B20%7D%3D%282%2A%28-19%29%2B19%2A2%29%2A10%3D%28-38%2B38%29%2A10%3D0)
Наверно 180-59=121(не точно)
(х-10)⁴-121=0
(х-10)⁴=121
х-10=±∜121
х-10=±∜11²
х-10=±√11
х-10=√11 х-10=-√11
х=√11+10 х=-√11+10
Ответ: х₁=√11+10, х₂=-√11+10.
(у-3)(у^2+3y+9)-y(y-4)(y+4) => (Раскрываем скобки) y^3+3y^2+9y-3y^2-9y-27-y^3-16y => -27-16y (Приводим подобные)