18. 用合理的方法计算。（每题2分，共12分）
$\frac{1}{6}+\frac{1}{3}-\frac{2}{9}$
$\frac{17}{24}-(\frac{7}{24}-\frac{7}{48})$
$8.37+\frac{2}{9}+11.63+\frac{7}{9}$
$\frac{7}{6}-\frac{11}{12}+\frac{1}{12}$
$\frac{5}{6}-\frac{1}{5}+\frac{1}{6}-\frac{3}{5}$
$9-5÷14-\frac{9}{14}$

18. $=\dfrac{3}{18}+\dfrac{6}{18}-\dfrac{4}{18}$
$=\dfrac{5}{18}$
$=\dfrac{17}{24}-\dfrac{7}{48}$
$=\dfrac{34}{48}-\dfrac{7}{48}$
$=\dfrac{9}{16}$
$=(8.37+11.63)+(\dfrac{2}{9}+\dfrac{7}{9})$
$=20+1$
$=21$
$=\dfrac{14}{12}-\dfrac{11}{12}+\dfrac{1}{12}$
$=\dfrac{1}{3}$
$=\dfrac{5}{6}+\dfrac{1}{6}-\dfrac{1}{5}-\dfrac{3}{5}$
$=(\dfrac{5}{6}+\dfrac{1}{6})-(\dfrac{1}{5}+\dfrac{3}{5})$
$=1-\dfrac{4}{5}$
$=\dfrac{1}{5}$
$=9-\dfrac{5}{14}-\dfrac{9}{14}$
$=9-(\dfrac{5}{14}+\dfrac{9}{14})$
$=9-1$
$=8$

19. 解方程。(每题2分,共6分)
$x+\dfrac{3}{4}=\dfrac{7}{8}$
$\dfrac{6}{7}+x+\dfrac{3}{7}=3$
$2x-0.5=0.3$

19. $x=\dfrac{1}{8}$；$x=\dfrac{12}{7}$；$x=0.4$

20. 求图形的表面积和体积。(单位:dm)(6 分)

20. $2×2=4(\mathrm{dm})$
挖去一个小正方体后,表面积不变,图形的表面积:$6×4×4=96(\mathrm{dm^2})$
图形的体积:$4×4×4-2×2×2=56(\mathrm{dm^3})$