Câu hỏi 28
Cho $n$ là một số nguyên dương chia hết cho $4$. Chứng minh rằng
$$\sin^2\left(1 \cdot \frac{360^\circ}{n}\right) + \sin^2\left(2 \cdot \frac{360^\circ}{n}\right) + \sin^2\left(3 \cdot \frac{360^\circ}{n}\right) + \dots + \sin^2\left(n \cdot \frac{360^\circ}{n}\right) = \frac{n}{2}.$$
$$\sin^2\left(1 \cdot \frac{360^\circ}{n}\right) + \sin^2\left(2 \cdot \frac{360^\circ}{n}\right) + \sin^2\left(3 \cdot \frac{360^\circ}{n}\right) + \dots + \sin^2\left(n \cdot \frac{360^\circ}{n}\right) = \frac{n}{2}.$$