cos 2x cos x – sin 2x sin x =0
cos(2x+x)=0
cos3x=0
3x=П/2+Пn
x=П/6+Пn/3
Ответ:П/6+Пn/3
A5 + a9 = 42
a3*a10 = 165
a1+ 4d + a1 + 8d = 42
(a1+ 2d)*(a1 + 9d) = 165
a1 + 6d = 21
(a1+ 2d)*(a1 + 9d) = 165
a1 = 21 - 6d
(21 - 6d + 2d)*(21 - 6d + 9d) = 165
a1 = 21 - 6d
(4d - 21) ( 3d + 21) = -165 **
**
12d^2 + 84d - 63d - 441 + 165 = 0
12d^2 + 21d - 276 = 0
4d^2 + 7d - 92 = 0
D = 49 + 1472 = 39^2
d1 = ( - 7 + 39)/8 = 4
d2 = ( - 7 - 39)/8 = - 5,75
a1 = 21 - 6*d = 21 - 6*4 = 21 - 24 = - 3
Ответ:
- 3; 4
<span>(x+3)(x+7)-(x+4)^2=0
x^2+3x+7x+21-x^2-8x-16=0
2x=-5
x=-5/2</span>
Х² = х
х² - х = 0
х ( х - 1 ) = 0
х = 0 и х - 1 = 0
х = 0 и х = 1.
Ответ : х = 0 ; 1.
Удачи))))