Tính giá trị biểu thức:
C=\(\dfrac{-1}{7}\)x\(\dfrac{5}{9}\)+\(\dfrac{-5}{9}\)x\(\dfrac{1}{7}\)+\(\dfrac{-5}{9}\)x\(\dfrac{5}{7}\)
D=\(\dfrac{4}{2x3}\)+\(\dfrac{4}{3x4}\)+\(\dfrac{4}{4x5}\)+....+\(\dfrac{4}{8x9}\)
A=\(\dfrac{5}{9}\)-\(\dfrac{5}{8}\)+\(\dfrac{2}{3}\)+\(\dfrac{4}{9}\)+\(\dfrac{-3}{8}\)+\(\dfrac{1}{3}\)
B=\(\dfrac{1}{2}\)+\(\dfrac{1}{6}\)+\(\dfrac{1}{12}\)+\(\dfrac{1}{20}\)+\(\dfrac{1}{30}\)+\(\dfrac{1}{42}\)+\(\dfrac{1}{56}\)