Cos 15= cos(45-30)= cos45cos30+sin45sin30= sqrt2/2*sqrt3/2 +sqrt2/2*1/2= (sqrt6+sqrt2)/4
Sin165=sin15
Sin15= sin(45-30)= sin45cos30-sin30cos 45=
(Sqrt6-sqrt2)/4
а) 6х^2-3х -42х-6х^2=90
-45х=90
Х=-2
б)4,5х+3х^2=3х^2+3х -30
4,5х -3х=-30
1,5х=-30
Х= 20
в)60х^2-35х-60х^2+44х=30+29х
-35х-29х+44х=30
Х=-1,5
г)24х+54х=-13+13х
65х= -13
Х=-0,2
1-27с^3=1-(3с)^3=(1-3c)(1-3c+9c^2)
Sin90=1, tg45=1
(2.1 - 1).3 = 3