y＝(xtanx)/(1＋x^2）y'=[(xtanx)'(1+x^2)-(xtanx)(1+x^2)']/(1＋x^2)^2=[tanx+x*(secx)^2](1+x^2)-(xtanx)(2x)]/(1＋x^2)^2=[(x^3)*(secx)^2-(x^2)*tanx+x*(secx)^2+tanx]/(1＋x^2)^2
