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N

nguyenbahiep1

[TEX]dk : sin x \not = 0 \\ 2cos^2x -1 + 1 + cot^2x = sinx + \frac{1}{sin^2x} \\ cos2x = sinx = cos(\frac{\pi}{2}-x) \\ 2x = \frac{\pi}{2}-x + k.2.\pi \\ 2x = x-\frac{\pi}{2}+ k.2.\pi [/TEX]
 
N

nguyencat92

=>Nghiệm chung là x=\prod_{i=1}^{n}/2 + k2\prod_{i=1}^{n}/3

x thuộc (2:40)=>0,7\leq k\leq18,9. k thuộc Z nên 1\leqk\leq18. =>tổng= 117\prod_{i=1}^{n}
 
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