【考点】正方形和等边三角形的性质;圆周角定理;锐角三角函数定义;特殊角的三角函数值;等腰直角三角形的判定和性质,特殊元素法的应用.
分析】如答图,连接AC,EC,AC与EF交于点M.
则根据对称性质,AC经过圆心O,
∴AC垂直 平分EF,∠EAC=∠FAC=1/2∠EAF=30°.
不妨设正方形ABCD的边长为2,则
.
∵AC是⊙O的直径,∴∠AEC=90°.
在RT△ADC中,
![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/8b13632762d0f703bf3056c701fa513d2697c52f?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
在RT△AEC中,
![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/8435e5dde71190effc8e50c4c71b9d16fdfa60b0?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
∵∠FEC=∠FAC=30°,
.
易知△GCH是等腰直角三角形,
![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/562c11dfa9ec8a134a709afbfe03918fa0ecc047?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
又∵△AEF是等边三角形,
![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/f31fbe096b63f6242de3f4188e44ebf81a4ca36b?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/f9198618367adab44767d65882d4b31c8701e472?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)