Tính: \(A=\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}.\)
\(A=\dfrac{1}{3}.\)\(A=\dfrac{1}{17}.\)\(A=\dfrac{1}{2}.\)\(\dfrac{1}{180}.\)Hướng dẫn giải:\(A=\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}\\ =\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}\\ =\dfrac{1}{2}-\dfrac{1}{6}=\dfrac{1}{3}.\)