作业帮求值：sin10sin30sin50sin70
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求值：sin10sin30sin50sin70
求值：sin10sin30sin50sin70
求值：sin10sin30sin50sin70
sin10sin30sin50sin70 =
1/2 * sin10 * sin50 * sin70 =
1/2 * sin10 * cos20 * cos 40 =
1/2 * cos10 * sin10 * cos20 * cos 40 / cos10 =
1/4 * sin20 * cos20 * cos40 / cos10 =
1/8 * sin40 * cos40 / cos10 =
1/16 * sin80 / cos10 =
1/16 * cos10 / cos10 =
1/16