7 xdx 例解 sec car secxd tanx secxtanx- tan xsecxdx secxtanx+ secxdx-secxd secx tanx+In(secx+ tanx)sec'xdx =sec'xdr=secx tanx+In(secx+tanx)+C 2 例8|sin"xdc(n∈N例 7 xdx  3 sec 解 xdx  3 sec  = sec xd tan x x x x xdx  = sec tan − tan sec 2   = x x + xdx − xdx 3 sec tan sec sec  = x x + x + x − xdx 3 sec tan ln(sec tan ) sec  xdx = x x + x + x + C  ln(sec tan ) 21 sec tan 21 sec3 例 8 sin xdx (n N) n  