trả lời:

B = 7x( 5/2x7 + 4/7x11 + 3/11x14 + 1/14x15 + 13/15x28 )

B = 7x(1/2 - 1/7 + 1/7 - 1/11 +....+ 1/15 - 1/28)

B = 7x(1/2 - 1/28)

B = 7x13/28

B = 13/4

Bàì làm:

\(B=\frac{5}{2.1}+\frac{4}{1.11}+\frac{3}{11.2}+\frac{1}{2.15}+\frac{13}{15.4}\)

\(B=7.\left(\frac{5}{2.7}+\frac{4}{7.11}+\frac{3}{11.14}+\frac{1}{14.15}+\frac{13}{15.28}\right)\)

\(B=7.\left(\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+\frac{1}{14}-\frac{1}{15}+\frac{1}{15}-\frac{1}{28}\right)\)

\(B=7.\left(\frac{1}{2}-\frac{1}{28}\right)\)

\(B=7.\frac{13}{28}\)

\(B=\frac{13}{4}\)

Bài làm:

\(B=\frac{5}{2.1}+\frac{4}{1.11}+\frac{3}{11.2}+\frac{1}{2.15}+\frac{13}{15.4}\)

\(B=7\left(\frac{5}{2.7}+\frac{4}{7.11}+\frac{3}{11.14}+\frac{1}{14.15}+\frac{13}{15.28}\right)\)

\(B=7\left(\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+\frac{1}{14}-\frac{1}{15}+\frac{1}{15}-\frac{1}{28}\right)\)

\(B=7\left(\frac{1}{2}-\frac{1}{28}\right)=7.\frac{13}{28}=\frac{13}{4}\)

Câu a là 1676250 nhé

1) \(A=20,11\times2,4+1,4\times20,11\times2-10,05\times10,4\) (tách 40,22 = 20,11 x 2)

       \(=20,11\times\left(2,4+1,4\times2\right)-10,05\times10,4\)

       \(=20,11\times5,2-10,05\times5,2\times2\) (tách 10,4 = 5,2 x 2)

        \(=5,2\times\left(20,11-20,1\right)\)

        \(=5,2\times0,01\)

        \(=0,052\)

5/2\(\times\)1=\(\frac{5}{2\times1}hay=\frac{5}{2}\times1\)

\(\left(1+1+1+1+1+1+1+1+1+1\right)=10.10x10=100\)

1+1+1+1+1+1+1+1+1+10=19

1+1+1+1+1+1+1+1+1+1.10

=19

đúng nhé