c) `( 3 1/2 : 21/32 - 8/21) . 4 5/7 + 2 7/8`
`= (7/2 . 32/21 - 8/21) . 33/7 + 23/8`
`=(16/3 - 8/21) . 33/7 + 23/8`
`=104/21 . 33/7 + 23/8`
`=1144/49 + 23/8`
`=10279/392`
d) `1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8`
`= 1/3 - 1/4 + 1/4 - 1/5 + 1/5-1/6+ 1/6 - 1/7 + 1/7 - 1/8`
`= 1/3 - 1/8`
`=5/24`
\(c,\left(3\dfrac{1}{2}:\dfrac{21}{32}-\dfrac{8}{21}\right).4\dfrac{5}{7}+2\dfrac{7}{8}\)
\(=\left(\dfrac{16}{3}-\dfrac{8}{21}\right).\dfrac{33}{7}+\dfrac{23}{8}\)
\(=\dfrac{104}{21}.\dfrac{33}{7}+\dfrac{23}{8}\)
\(=\dfrac{1144}{49}+\dfrac{23}{8}=\dfrac{10279}{392}\)
\(d,\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}\)
\(=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}\)
\(=\dfrac{1}{3}-\dfrac{1}{8}=\dfrac{5}{24}\)