вычислите cos2a+3cos^2a если a= -2/корень из 5
вычислите если
A=cos 2a=(cos a)^2-(sin a)^2==(sin a)^2*((ctg a)^2-1)=(sin a)^2*(4-1)=3(sin a)^21+(ctg a)^2=1/(sin a)^2, 5=1/(sin a)^2.(sin a)^2=1/5.A=3/5
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A=cos 2a=(cos a)^2-(sin a)^2=
=(sin a)^2*((ctg a)^2-1)=(sin a)^2*(4-1)=3(sin a)^2
1+(ctg a)^2=1/(sin a)^2, 5=1/(sin a)^2.
(sin a)^2=1/5.
A=3/5