令x² = tanz => 2xdx = sec²zdz当x = 1,z = π/4；当x = 2,z = arctan(4)∫(1~2) 2x/√(1 + x^4) dx= ∫(π/4~arctan(4)) 2x/√(1 + tan²z) * sec²z/(2x) dz= ∫(π/4~arctan(4)) sec²z/√...