无界令x = 1/(2kπ + π/2) k = 1 ,2 ,3 .此时x是趋于0的所以f(x)= (2kπ + π/2) * sin(2kπ + π/2) = (2kπ + π/2) 趋于正无穷 所以f(x)无上界再令x = 1/(2kπ - π/2) k = 1 ,2 ,3 .此时x是趋于0的所以f(x)= (2...