Tính nhanh B=5/2.1+4/1.11+3/11.2+1/2.15+13/15.4

Tính nhanh B=5/2.1+4/1.11+3/11.2+1/2.15+13/15.4

Giải thích các bước giải:

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

$\frac{B}{7}$ = $\frac{5}{2.7}$ + $\frac{4}{7.11}$ + $\frac{3}{11.14}$ + $\frac{1}{14.15}$ + $\frac{13}{15.4}$ 

$\frac{B}{7}$ = $\frac{1}{2}$ - $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{11}$ + ... + $\frac{1}{15}$ - $\frac{1}{28}$

$\frac{B}{7}$ = $\frac{1}{2}$ - $\frac{1}{28}$

$\frac{B}{7}$ = $\frac{13}{28}$ 

B = $\frac{13}{28}$ x 7 

B = $\frac{13}{4}$