C) \( C = \frac{cos \alpha + sin^3 \alpha}{cos^2 \alpha - sin \alpha} \), biết \( cot \alpha = \sqrt{2} \) 5. Cho \( \triangle ABC \), chứng minh: a) \( sin A = sin(B + C) \) b) \( cos A = -cos(B + C) \) c) \( cos^2 \frac{A}{2} = sin \frac{B + C}{2} \) d) \( tan(A + B) + tan C = 0 \) e) \( \frac{A}{2} = sin \frac{B + C}{2} \) f) \( tan \frac{A}{2} = cot \frac{B + C}{2} \)

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C) \( C = \frac{cos \alpha + sin^3 \alpha}{cos^2 \alpha - sin \alpha} \), biết \( cot \alpha = \sqrt{2} \)

5. Cho \( \triangle ABC \), chứng minh:
a) \( sin A = sin(B + C) \)
b) \( cos A = -cos(B + C) \)
c) \( cos^2 \frac{A}{2} = sin \frac{B + C}{2} \)
d) \( tan(A + B) + tan C = 0 \)
e) \( \frac{A}{2} = sin \frac{B + C}{2} \)
f) \( tan \frac{A}{2} = cot \frac{B + C}{2} \)