X² + x² - 20x + 5x + 13 = 0
2x² - 15x + 13 = 0
D = b² - 4ac = 225 - 8×13 = 121 = 11²
x1 = ( 15 + 11) /4 = 6,5
x2 = ( 15 - 11)/4 = 1
2(n-1)! / (n-3)! - n = 8
n! = 1*2*3*...*n
n>=3 n∈N
2 * 1*2*3*...*(n-3)*(n-2)*(n-1) / 1*2*3*....*(n-3) - n = 8
2*(n-2)(n-1) - n - 8 = 0
2(n² - 2n - n + 2) - n - 8 = 0
2n² - 6n + 4 - n - 8 = 0
2n² -7n - 4 = 0
D=49 + 4*4*2 = 81
n12= (7 +- 9)/4 = - 1/2 4
n = 4
Проверка 2*3!/1! - 4 = 2*6 - 4 = 8
1)
3 (5-x)=11+2x
15-3x=11+2x
-3x-2x= 11-15
-5x=-4
X= -4÷(-5)
X=0,8
Решение:
5,39с²-2,6с²-1с⁷=2,79с²-с⁷.