D= (-1)^2-4*5*(-1)=1+20=21>0- 2 корня
x1.2= -(-1)+- <span>√21 /10 (дробью)
x1= 1+</span>5 - √21/10(дробью) = примерно 0.55826
x2=1-5 - √21 /10 (дробью) = примерно <span>-0.35826</span>
Ответ:
надеюсь, что все правильно ;)
99,9% ,то что правильно
в номере б) на фотографии не поместилось
х>4
х<2
х€(перечеркнутый 0)
![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)
=2sin(2*7pi/12)=2sin(7pi/6)=2*(-1/2)=-1
формула синуса двойного аргумента.
4n²-p²=(2n)²-p²=(2n-p)(2n+p)
8n+4p=4(2n+p)
(4n²-p²)/(4(2n+p))=(2n-p)(2n+p)/4(2n+p)=(2n-p)/4
О т в е т. (2n-p)/4