√3.cos(7π/6)+cos(-480°)=√3.cos(210°)+cos(480°)=
=√3.cos(180°+30°)+cos(360°+120°)=√3.(-cos30°)+cos120°=
=√3.(-√3/2)-cos60°=-3/2-1/2=-4/2=-2
А21-а10=(а1+20d)-(a1+9d)=11d=30-(-3)=33
11d=33
d=3
a1=a10-9d=-3-27=-30
(-1)в квадрате+(12корень из2)в квадрате=1+144*2=289
15/10=1/3x
3/2=1/3x
9x=2
x=2/9