\(\dfrac{b-a}{a\cdot b}\)= \(\dfrac{1}{a}\)-\(\dfrac{1}{b}\)

=\(\dfrac{2}{35}\)+\(\dfrac{4}{77}\)+\(\dfrac{2}{143}\)+\(\dfrac{4}{221}\)+\(\dfrac{2}{323}\)+\(\dfrac{4}{437}\)+\(\dfrac{2}{575}\)

=\(\dfrac{7-5}{5\cdot7}\)+\(\dfrac{11-7}{7\cdot11}\)+\(\dfrac{13-11}{11\cdot13}\)+\(\dfrac{17-13}{13\cdot17}\)+\(\dfrac{19-17}{17\cdot19}\)+\(\dfrac{21-19}{19\cdot21}\)+\(\dfrac{23-19}{19\cdot23}\)

=\(\dfrac{1}{5}\)-\(\dfrac{1}{7}\)+\(\dfrac{1}{7}\)-\(\dfrac{1}{11}\)+\(\dfrac{1}{11}\)-\(\dfrac{1}{13}\)+\(\dfrac{1}{13}\)-\(\dfrac{1}{15}\)+\(\dfrac{1}{15}\)-\(\dfrac{1}{17}\)+\(\dfrac{1}{17}\)-\(\dfrac{1}{19}\)+\(\dfrac{1}{19}\)-\(\dfrac{1}{21}\)+\(\dfrac{1}{21}\)-\(\dfrac{1}{23}\)

=\(\dfrac{1}{5}\)-\(\dfrac{1}{23}\)