tan0=0 cot0=0 sin0=0 cos0=1 csc0=0 sec0=1
tanπ/2无意义 cotπ/2无意义 sinπ/2=1 cosπ/2=0 cscπ/2=1 secπ/2=0
tanπ=0 cotπ=0 sinπ=0 cosπ=1 cscπ=0 secπ=1