计算原则：先算小括号,然后算中括号.
21/22*[(5/6-4/9)/7/4]
=21/22*[21/54/7/4]
=21/22*[(7*3)/(3*18*7*4)]
=21/22*[1/(18*4)]
=21/22*(1/72)
=21/(22*72)
=(7*3)/(22*24*3)
=7/(22*24)
=7/528