\(a.2\times999999\times5=999999\times\left(2\times5\right)\)
\(=999999\times10=9999990\)
\(b.840\div2\div5=840\div\left(2\times5\right)\)
\(=840\div10=84\)
a)2.999999.5
=2.5.999999
=10.999999
=9999990
b)840:2:5
=840:(2.5)
=840:10
=84
chúc bạn học tốt !!!