Question

Cos 2a, если cos a =-2/9​

Answer 1

Ответ:

$sina=\sqrt{1-\frac{4}{81} } =\sqrt{\frac{81-4}{81} } =\sqrt{\frac{77}{81} } =\frac{\sqrt{77} }{9} \\\\\frac{4}{81} -\frac{77}{81} =\frac{4-77}{81} =\frac{73}{81}$

Объяснение:

Answer 2

To find the value of cos 2a, we can use the double-angle formula for cosine, which states:

cos 2a = 2 * cos^2(a) - 1

Given that cos a = -2/9, we can substitute this value into the formula:

cos 2a = 2 * (-2/9)^2 - 1 cos 2a = 2 * (4/81) - 1 cos 2a = 8/81 - 1 cos 2a = (8 - 81) / 81 cos 2a = -73 / 81

So, the value of cos 2a is -73/81.