tan(arctan1/5 + arctan3)
=(tan(arctan1) + tan(arctan3)) /( 1-tan(arctan1/5) tan(arctan3))
=(1/5+3) / (1-1/5*3)
=8
2注意：勾3股4弦5
arctan(3/4)=arccos(4/根号(3*3+4*4))=arccos(4/5)……（1）
arctan(3/4)=arcsin(3/5)……………………………………（2）
cos[2archtan(3/4)]=cos[archtan(3/4)]*cos[archtan(3/4)]-sin[archtan(3/4)]*sin[archtan(3/4)]………………(3)
将（1）和（2）代入（3）即得到结果.