Cho ab+ac+bc=1 . Chứng minh: \frac{1}{ab} +\frac{1}{ac} + \frac{1}{bc} \geq 3+ \sqrt{\frac{(a+b)(a+c)}{a^{2}}} +\sqrt{\frac{(b+c)(c+a)}{b^{2}}} +\sqrt{\frac{(c+a)(c+b)}{c^{2}}}