6) =4cosa-cosa=3cosa; 3*(-0,9)=-2,7
7)sin^2 a+cos^2 a=1
sina=+√(1-cos^2 a), 2-ая четверть
sina=√(1-(-√11/15)^2)=√(1-11/15)=√(4/15)=2/√15;
√15 sina=√15 *(2/√15)=2
8) (2√3(sin50 sin(90+10)+cos50si10)) /(cos40 cos(90+10) + sin40 cos10)=
(2√3(sin50 cos10+cos50sin10)) / (-cos40 sin10 +sin40 cos10)=
=(2√3 sin(50+10)) /(sin(40-10))=(2√3sin60) / (sin30)=(2√3 * √3/2) / (1/2)=3*2=6;
9) √7 tga (-cosa)=√7 (sina/cosa) *(-cosa)=-√7sina;
sina=√7/5;
-√7*(√7/5)=-7/5=-1,4
10)2√3sin(6π+π/3) *sin(3π-π/6)=2√3sin(π/3) *sin(π/6)=2√3 * (√3/2)*1/2)=
=(2*3)/4=3/2=1,5
11)1/cos^2x=1+tg^2x;получим
1+tg^2 x+cosx=tg^2 x
cosx=-1; x=π+2πn, n-целое
12)sin(π+x)=-sinx, тогда
-sinx=cosπ
-sinx=-1
sinx=1; x=π/2+2πn