Hệ số: -10

\(1-5-9+13+17-21-25+...-393+397+401\)

\(=\left(1-5-9+13\right)+\left(17-21-25+29\right)+...+\left(385-389-393+397\right)+401\)

\(=0+0+...+0+401=401\)

I sometimes have lunch at school

1.A

2.C

\(\dfrac{111}{110}\times\dfrac{112}{111}\times\dfrac{113}{112}\times\dfrac{114}{113}\times\dfrac{55}{57}=\dfrac{111\times112\times113\times114}{110\times111\times112\times113}\times\dfrac{55}{57}\)

                                                   \(=\dfrac{114}{110}\times\dfrac{55}{57}=\dfrac{57}{55}\times\dfrac{55}{57}=1\)

\(y:1,5+5,75=10,55\)

\(\Leftrightarrow y:1,5=10,55-5,75\)

\(\Leftrightarrow y:1,5=4,8\)

\(\Leftrightarrow y=4,8\times1,5\)

\(\Leftrightarrow y=7,2\)