Cho hàm số \(y = f\left( x \right) = 2{\sin ^2}x - 5\).

\(\sin \left( {x + \frac{\pi }{4}} \right) = 0\)\( \Leftrightarrow x + \frac{\pi }{4} = k\pi \)\( \Leftrightarrow x =  - \frac{\pi }{4} + k\pi ,k \in \mathbb{Z}\).

Vì \(x \in \left[ {0;2025\pi } \right]\) nên \(0 \le  - \frac{\pi }{4} + k\pi  \le 2025\pi \)\( \Leftrightarrow \frac{1}{4} \le k \le \frac{{8101}}{4}\) mà \(k \in \mathbb{Z}\) nên \(k \in \left\{ {1;2;..;2025} \right\}\).

Khi đó \(S = \frac{{3\pi }}{4} + \frac{{7\pi }}{4} + \frac{{11\pi }}{4} + ... + \frac{{8099\pi }}{4}\)\( = \frac{\pi }{4}\left( {3 + 7 + 11 + ... + 8099} \right)\)\( = \frac{\pi }{4}.\frac{{\left( {3 + 8099} \right).2025}}{2} = \frac{{4051.2025\pi }}{4}\).

Khi đó \(\frac{{4S}}{{2025\pi }} = \frac{4}{{2025\pi }}.\frac{{4051.2025\pi }}{4} = 4051\).

Trả lời: 4051.

a) b)  Vì \(\frac{{3\pi }}{2} < x < 2\pi \) nên \(\sin x < 0;\cos x > 0\).

Ta có \(1 + {\cot ^2}x = \frac{1}{{{{\sin }^2}x}}\)\( \Rightarrow \frac{1}{{{{\sin }^2}x}} = 1 + {\left( { - \sqrt 3 } \right)^2} = 4\)\( \Rightarrow {\sin ^2}x = \frac{1}{4}\)\( \Rightarrow \sin x =  - \frac{1}{2}\).

Ta có \(\cos x = \cot x.\sin x = \left( { - \sqrt 3 } \right).\left( { - \frac{1}{2}} \right) = \frac{{\sqrt 3 }}{2}\).

c) \(\sin \left( {\frac{{4\pi }}{3} - x} \right) = \sin \frac{{4\pi }}{3}\cos x - \cos \frac{{4\pi }}{3}\sin x\)\( = \left( { - \frac{{\sqrt 3 }}{2}} \right).\frac{{\sqrt 3 }}{2} - \left( { - \frac{1}{2}} \right)\left( { - \frac{1}{2}} \right) = \frac{{ - 3}}{4} - \frac{1}{4} =  - 1\).

d) Vì \(\cot x =  - \sqrt 3 \)\( \Rightarrow \tan x =  - \frac{1}{{\sqrt 3 }}\).

\(\tan \left( {x + \frac{\pi }{3}} \right) = \frac{{\tan x + \tan \frac{\pi }{3}}}{{1 - \tan x\tan \frac{\pi }{3}}}\)\( = \frac{{ - \frac{1}{{\sqrt 3 }} + \sqrt 3 }}{{1 - \left( { - \frac{1}{{\sqrt 3 }}} \right).\sqrt 3 }}\)\( = \frac{2}{{2\sqrt 3 }} = \frac{1}{{\sqrt 3 }}\).

Đáp án: a) Sai;   b) Sai;   c) Sai;   d) Đúng.