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如图：∠AOC=2∠ABC（同弧所对的圆心角等于圆周角的2倍）...(1)∵∠0AC=∠0CA∴∠AOC=180-∠0AC-∠0CA=180-2∠0AC................(2)由(1)(2):----∠ABC=90-∠0AC∵AD是⊙0的切线，∴DA⊥OA,---∠DAC=90-∠0AC=∠ABC∴弦切角等于弦所对的圆周角
如图：∠AOC=2∠ABC（同弧所对的圆心角等于圆周角的2倍）...(1)∵∠0AC=∠0CA∴∠AOC=180-∠0AC-∠0CA=180-2∠0AC................(2)由(1)(2):----∠ABC=90-∠0AC∵AD是⊙0的切线，∴DA⊥OA,---∠DAC=90-∠0AC=∠ABC∴弦切角等于弦所对的圆周角