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发表于 2020-9-22 14:16:31
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暴力一下好了,$180*180$, 答案是$8$个,
- Select[Flatten[Table[{i ,j},{i,1,180-1},{j,1,180-1}],1]/180 Pi,Function[{a,b},TrigReduce[Sin[a+b]Sin[b]-Sin[a]Sin[a+2b]]==0]@@#&]
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本题有$8$组解
\[\left\{\frac{\pi }{10},\frac{\pi }{6}\right\},\left\{\frac{\pi }{10},\frac{5 \pi }{6}\right\},\left\{\frac{\pi }{6},\frac{3 \pi }{10}\right\},\left\{\frac{\pi }{6},\frac{7 \pi }{10}\right\},\left\{\frac{5 \pi }{6},\frac{3 \pi }{10}\right\},\left\{\frac{5 \pi }{6},\frac{7 \pi }{10}\right\},\left\{\frac{9 \pi }{10},\frac{\pi }{6}\right\},\left\{\frac{9 \pi }{10},\frac{5 \pi }{6}\right\}\]
对应知乎的原题有$12$组解
\[\left\{\frac{\pi }{15},\frac{\pi }{10}\right\},\left\{\frac{2 \pi }{15},\frac{\pi }{6}\right\},\left\{\frac{\pi }{5},\frac{\pi }{5}\right\},\left\{\frac{4 \pi }{15},\frac{5 \pi }{6}\right\},\left\{\frac{2 \pi }{5},\frac{\pi }{5}\right\},\left\{\frac{7 \pi }{15},\frac{7 \pi }{10}\right\},\left\{\frac{8 \pi }{15},\frac{\pi }{6}\right\},\left\{\frac{3 \pi }{5},\frac{3 \pi }{5}\right\},\left\{\frac{11 \pi }{15},\frac{\pi }{10}\right\},\left\{\frac{4 \pi }{5},\frac{2 \pi }{5}\right\},\left\{\frac{13 \pi }{15},\frac{3 \pi }{10}\right\},\left\{\frac{14 \pi }{15},\frac{\pi }{6}\right\}\]
\[\{12 {}^{\circ},18 {}^{\circ}\},\{24 {}^{\circ},30 {}^{\circ}\},\{36 {}^{\circ},36 {}^{\circ}\},\{48 {}^{\circ},150 {}^{\circ}\},\{72 {}^{\circ},36 {}^{\circ}\},\{84 {}^{\circ},126 {}^{\circ}\},\{96 {}^{\circ},30 {}^{\circ}\},\{108 {}^{\circ},108 {}^{\circ}\},\{132 {}^{\circ},18 {}^{\circ}\},\{144 {}^{\circ},72 {}^{\circ}\},\{156 {}^{\circ},54 {}^{\circ}\},\{168 {}^{\circ},30 {}^{\circ}\}\}\]
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