变量代换 x＝2sinu，dx＝2cosudu，原式＝∫2(cos² u)/sinu du＝ ∫[2(1-sin² u)/sinu du= 2∫(1/sinu - sinu)du= 2∫(cscu-sinu)du= -2ln|cscu+cotu|-2cosu+C＝ 2ln|x|-2ln[√(4-x²)+2]+√(4-x²)+C
