若sin^2 40度+sin^2 a=1,且a为锐角,则a=?
人气：298 ℃ 时间：2020-10-02 00:25:59
解答
sin²40º + sin²a = 1
sin²a = 1 - sin²40º = cos²40º = sin²50º
sin²a = sin²50º
½(1 - cos2a) = ½(1 - cos100º)
cos2a = cos100º
2a = 360ºk ± 100º
a = 180ºk ± 50º
0º < a < 90º
a = 50º
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