1) 2а = 5*pi\6
3a = 15pi\12 = 4\3 *pi
cos(4\3 * pi) = cos(pi + pi\3) = -cos(pi\3) = -1\2
2) 2a = pi\6
a = pi\12
3a = pi\4
cos(pi\4) = sqrt(2)\2
<span>
3√5/4*5=3√5/20</span>
<span /><span>ответ:3√5/20</span>
5a ( a^2 - 4a) -4a (a^2 -5a)=0
5a^3-20a^2 - 4a^3+20a^2 =0
a^3=0
a=0