Question 1152666
.
<pre>

cos^2 (𝛑/9)         + cos^2 (2𝛑/9)       + cos^2 (3𝛑/9)       + cos^2 (4𝛑/9) = 


= (1 - sin^2 (𝛑/9)) + (1 - sin^2 (2𝛑/9)) + (1 - sin^2 (3𝛑/9)) + (1 - sin^2 (4𝛑/9)) = 


= 4 - (sin^2 (𝛑/9) + sin^2 (2𝛑/9) + sin^2 (3𝛑/9) + sin^2 (4𝛑/9)) = {{{4 - 9/4}}} = {{{(16-9)/4}}} = {{{7/4}}}.    <U>ANSWER</U>
</pre>