Đáp án: $Q=\dfrac12$
Giải thích các bước giải:
Ta có:
$Q=\cos^225^o-\cos^235^o+\cos^245^o-\cos^255^o+\cos^265^o$
$\to Q=(\cos^225^o+\cos^265^o)-(\cos^235^o+\cos^255^o)+\cos^245^o$
$\to Q=(\cos^225^o+\sin^2(90^o-65^o))-(\cos^235^o+\sin^2(90^o-55^o))+\cos^245^o$
$\to Q=(\cos^225^o+\sin^225^o)-(\cos^235^o+\sin^235^o)+\cos^245^o$
$\to Q=1-1+\dfrac12$
$\to Q=\dfrac12$
