Giải thích các bước giải:
b.Ta có:
$B=\dfrac{3}{5.7}+\dfrac{3}{7.9}+...+\dfrac{3}{59.61}$
$\to B=\dfrac32(\dfrac{2}{5.7}+\dfrac{2}{7.9}+...+\dfrac{2}{59.61})$
$\to B=\dfrac32(\dfrac{7-5}{5.7}+\dfrac{9-7}{7.9}+...+\dfrac{61-59}{59.61})$
$\to B=\dfrac32(\dfrac15-\dfrac17+\dfrac17-\dfrac19+...+\dfrac1{59}-\dfrac1{61})$
$\to B=\dfrac32(\dfrac15-\dfrac1{61})$
$\to B=\dfrac{168}{710}$
c.Ta có:
$C=\dfrac{\dfrac5{22}+\dfrac3{13}-\dfrac12}{\dfrac4{13}-\dfrac2{11}+\dfrac32}$
$\to C=\dfrac{-\dfrac{6}{143}}{\dfrac{465}{286}}$
$\to C=-\dfrac4{155}$
