原式通分后=[(2a/(2a-1)-1/(2a(2a-1))]÷[(4a+4a²+1)/4a]=[(4a²-1)/2a(2a-1)]÷[(2a+1)²/4a]=[(2a-1)(2a+1)/2a(2a-1)]×4a/(2a+1)²约分后得[(2a+1)/2a]×4a/(2a+1)²=2/(2a+1)
