a) Rút gọn biểu thức \(A=\frac{1}{1+\sqrt{2}}+\frac{1}{\sqrt{2}+\sqrt{3}}+...+\frac{1}{\sqrt{2017}+\sqrt{2018}}.\)
b) Chứng minh rằng \(1+\frac{1}{\sqrt{2}}+\frac{1}{\sqrt{3}}+...+\frac{1}{\sqrt{2017}}>2\left( \sqrt{2018}-1 \right)\).
A.a) \(A=\sqrt{2016}-2\)
B.a) \(A=\sqrt{2017}-1\)
C.a) \(A=\sqrt{2018}-1\)
D.a) \(A=\sqrt{2018}-1\)