j) (8,9+ 65,72) - (65,72 - 1,1)
= 74,62 - 64.62 = 10
k) \(2\dfrac{3}{7}+1\dfrac{4}{7}=\dfrac{17}{7}+\dfrac{11}{7}=\dfrac{28}{7}=4\)
l) \(A=\dfrac{1}{3\cdot5}+\dfrac{1}{5\cdot7}+\dfrac{1}{7\cdot9}+...+\dfrac{1}{37\cdot39}\)
\(=\dfrac{1}{2}\left(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{37}-\dfrac{1}{39}\right)\)
\(=\dfrac{1}{2}\left(\dfrac{1}{3}-\dfrac{1}{39}\right)\)
\(=\dfrac{1}{2}\cdot\dfrac{4}{13}=\dfrac{2}{13}\)