1)
- 8 * ( - 7 1/2) * (4х - 3,6) = 0
- 8 * (-7,5) * (4х - 3,6) = 0
60 * (4х - 3,6) = 0 |: 60
4х - 3,6 = 0
4х = 3,6
х = 3,6 : 4
х= 0,9
проверим:
-8 * ( - 7 1/2 ) * ( 4*0,9 - 3,6) = 0
60 * 0 = 0
0 = 0
х + 2х + ... + 20х = 2100
Арифметическая прогрессия Sn = [(a₁+a n) / 2 ] * n
а₁=х
а n = 20x
n = 20
Sn = 2100
( (x +20x)/2 ) * 20 = 2100
10(x + 20x) = 2100
10x + 200x = 2100
210x = 2100
x= 2100 : 210
х = 10