(2) AD/AC=sin30/sin130=1/2*1/sin130
sin130*sin40/sin80=1/2*(cos(130-40)-cos(130+40))/sin80=1/2*(cos90-cos170)/sin80=1/2*(0-cos(90+80))/sin80=1/2*sin80/sin80=1/2
=>1/2*1/sin130=sin40/sin80
=>(1)=(2)
...
(2) AD/AC=sin30/sin130=1/2*1/sin130
sin130*sin40/sin80=1/2*(cos(130-40)-cos(130+40))/sin80=1/2*(cos90-cos170)/sin80=1/2*(0-cos(90+80))/sin80=1/2*sin80/sin80=1/2
=>1/2*1/sin130=sin40/sin80
=>(1)=(2)
...
• 或者,直接利用三角公式列方程求出X :) -网恋无罪- ♂ (411 bytes) () 11/16/2011 postreply 15:39:41
• 要求不用三角公式而用初等几何的方法解题. -redplum- ♀ (131 bytes) () 11/16/2011 postreply 19:27:35