求证：同弦所对的圆心角是圆周角的2倍哟

如下图所示：2(x+y+z)=180°

x+y+z=90°

∴x=90°-y-z

∠BDC=180°-2x=180°-2(90°-y-z)=180°-180°+2(y+z)=2(y+z)=2∠BAC