Các bạn nhớ ghi cách làm ra nhé!

a)85.(35-27)-35.(85-27)

=85.35-27-35.85-27

=(85-85).(35-35).(27-27)

=0.0.0

=0

b)47.(45-15)-47.(45+15)

=47.45-15-47.45+15

=(47-47).(45-45).(15+15)

=0.0.30

=0

a) - 1350

b) - 141o

\(B=\dfrac{1}{3}+\dfrac{1}{15}+\dfrac{1}{35}+...+\dfrac{1}{97\times99}\)

\(=\dfrac{1}{1.3}+\dfrac{1}{3.5}+\dfrac{1}{5.7}+...+\dfrac{1}{97.99}\)

\(=\dfrac{1}{2}.\left(\dfrac{2}{1.3}+\dfrac{2}{3.5}+\dfrac{2}{5.7}+...+\dfrac{2}{97.99}\right)\)

\(=\dfrac{1}{2}.\left(1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{97}-\dfrac{1}{99}\right)\)

\(=\dfrac{1}{2}.\left(1-\dfrac{1}{99}\right)\)

\(=\dfrac{1}{2}.\dfrac{98}{99}\)

\(=\dfrac{49}{99}\)

dấu \(.\) là dấu \(\times\)

sai bét tè lét

(15x18x24):(5x9x12)

=6480:540

=12

=6480:540

=12

tích mình đi

ai tích mình

mình ko tích lại đâu

thanks

• \(B=\frac{1}{1.5}+\frac{1}{5.9}+\frac{1}{9.13}+...+\frac{1}{93.97}\)

           \(4.B=\frac{4}{1.5}+\frac{4}{5.9}+\frac{4}{9.13}+...+\frac{4}{93.97}\) 

            \(4.B=1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+...+\frac{1}{93}-\frac{1}{97}\)

            \(4.B=1-\frac{1}{97}\)

             \(4.B=\frac{96}{97}\)

                 \(B=\frac{96}{97}:4\)

                 \(B=\frac{24}{97}\)