√(10 - 2√21) = √[(√7)² - 2√7*√3 +(√3)²] = √(√7 - √3)² = √7 - √3
Решие
<span>sin α/2 =2/√7 , 0<α<п
sin</span>²(α/2) = (1 - cosα)/2
(2/√7)² = (1 - cosα)/2
1 - cosα = 8/7
cosα = 1 - 8/7
cosα = - 1/7
sinα = √(1 - cos²α) = √(1 - (- 1/7)²) = √(1 - 1/49) = √(48/49) =
<span>= 4√3 / 7</span>
X²-7x-18=0
D=49+72= 121
x1=(7+11)/2=9
x2=(7-11)/2= -2