3х²+bx+12=0
D=b²-4*3*12=b²-144 < 0
b²-144<0
(b-12)(b+12)<0
+ - +
-------- -12---------- 12 ----------
\\\\\\\\\\\
-12 < b < 12
Давай по частям сделаем:
1) Sin420° = Sin(360° + 60°) =Sin60° = √3/2
2)Сos600° = Cos(180°*3 + 60°) = -Cos60° = -1/2
3) √3/2 * (-1/2) = -√3/4
X^2 - 5x + 6 = 0
D = b^2 - 4ac
D = -5^2 - 4 * 1 * 6 = 25 - 24
D = 1
x1 = 5 + 1 / 2 = 3
x2 = 5 - 1 / 2 = 2
Ответ : x1 = 3 ; x2 = 2
5x^2 - 9x - 2 = 0
D = b^2 - 4ac
D = -9^2 - 4 * 5 * (-2) = 81 + 40 = 121
D = 11
x1 = 9 + 11 /2 = 9
x2 = 9 - 11 / 2 = -1
Ответ : x1 = 9 ; x2 = -1