证明：( 21n+4)/(14n+3)=1+(7n+1)/(14n+3).
又因为(14n+3)/(7n+1)=2+1/(7n+1).
则1/(7n+1)不可约.所以(14n+3)/(7n+1)不可约,
所以(21n+4)/(14n+3)也是不可约的.
即：
(21n+4,14n+3)=1