1+2cos2x=0时x为多少？ COS x=1/2那么COS2x等于多少
1+cos2x=\u591a\u5c11\uff1f2cos²x
\u89e3\u91ca\uff1a
1 + cos2x = 1 + \uff08cos²x - sin²x\uff09\uff08\u4e8c\u500d\u89d2\u516c\u5f0f\uff09
= 1 + \uff08cos²x - 1 + cos²x \uff09
= 1 + \uff082cos²x-1\uff09
= 2cos²x
\u6269\u5c55\u5185\u5bb9\uff1a
\u4e8c\u500d\u89d2\u516c\u5f0f\uff1a
\u76f4\u63a5\u7528\u53cd\u4e09\u89d2\u51fd\u6570 cosx=1/2 arccos1/2=60\u00b0 x=60\u00b0\uff0c2x=120\u00b0 cos120\u00b0=-1/2
\u7528\u4e8c\u500d\u89d2\u516c\u5f0f\uff0ccos2x=2cos²x-1=1/2-1=-1/2
\u671b\u91c7\u7eb3\u54af\uff01
两边除以2得cos2x=-1/2;
2x=2pi/3.
x=pi/3.
1+2cos2x=0
2cos2x=-1
x=90⁰
…………………x=π/3………………
解如下图所示
绛旓細涓よ竟闄や互2寰cos2x=-1/2;2x=2pi/3.x=pi/3.
绛旓細鈭祒鈭圼0,蟺]鈭2x鈭圼0,2蟺]cos2x>=-1/2 鎵浠,0=
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