求arctan(1/5)＋arctan(2/3)tan[arctan(1/5)+arctan(2/3)]==[tanarctan(1/5)+tanarctan(2/3)]/[1-tanarctan(1/5)tanarctan(2/3)]=[(1/5)+(2/3)]/[1-(1/5)(2/3)]=(13/15)/(13/15)=1