解如下图所示
y= tanxy'= (secx)^2y''= 2(secx)^2.tanx=2[1+(tanx)^2]. tanx = 2tanx +2(tanx)^3y'''= 2(secx)^2 + 6(tanx)^2 . (secx)^2 = 2+8(tanx)^2 + 6(tanx)^4y''''=16(tanx).(secx)^2 + 24(tanx)^3. (secx)^2=16tanx + 40(tanx)^3 + 24(tanx)^5