Question

Tính tổng sau:

A=\(\dfrac{20}{1.6}+\dfrac{20}{6.11}+........+\dfrac{20}{51.56}+\dfrac{20}{56.61}\)

Answer 1

\(A=\dfrac{20}{1.6}+\dfrac{20}{6.11}+.........+\dfrac{20}{51.56}+\dfrac{20}{56.61}\)

\(\dfrac{1}{4}A=\dfrac{5}{1.6}+\dfrac{5}{6.11}+........+\dfrac{5}{51.56}+\dfrac{5}{56.61}\)

\(\dfrac{1}{4}A=1-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{11}+........+\dfrac{1}{51}-\dfrac{1}{56}+\dfrac{1}{56}-\dfrac{1}{61}\)

\(\dfrac{1}{4}A=1-\dfrac{1}{61}\)

\(\Rightarrow A=\dfrac{60}{61}:\dfrac{1}{4}\)

\(\Rightarrow A=\dfrac{60}{61}.4\)

\(\Rightarrow A=\dfrac{240}{61}\)