Question

Giúp mk câu 4 nha

Answer 1

bị lấp bn ạ

Answer 2

Ta có: \(A=\dfrac{2}{42}+\dfrac{2}{56}+\dfrac{2}{72}+\dfrac{2}{90}+...+\dfrac{2}{420}\)

=>\(A=2.\left(\dfrac{1}{6.7}+\dfrac{1}{7.8}+\dfrac{1}{8.9}+\dfrac{1}{9.10}+...+\dfrac{1}{20.21}\right)\)

=>\(A=2\left(\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+...+\dfrac{1}{20}-\dfrac{1}{21}\right)\)

=>\(A=2.\left(\dfrac{1}{6}-\dfrac{1}{20}\right)\)

=>\(A=\dfrac{1}{3}-\dfrac{1}{10}\)

=>\(A=\dfrac{7}{30}\)