Đáp án:
a/$\dfrac{3}{4}$, $\dfrac{5}{6}$
Giải thích các bước giải:
a/$\dfrac{1}{1}$.$\dfrac{1}{2}$+$\dfrac{1}{2}$.$\dfrac{1}{3}$+$\dfrac{1}{3}$.
$\dfrac{1}{4}$+$\dfrac{1}{4}$.$\dfrac{1}{5}$
=$\dfrac{1}{1.2}$+$\dfrac{1}{2.3}$+$\dfrac{1}{3.4}$+$\dfrac{1}{4.5}$
=$\dfrac{1}{1}$-$\dfrac{1}{2}$+$\dfrac{1}{2}$-$\dfrac{1}{3}$+$\dfrac{1}{3}$-$\dfrac{1}{4}$+$\dfrac{1}{4}$-$\dfrac{1}{4}$
=1-$\dfrac{1}{4}$
=$\dfrac{3}{4}$
b/$\frac{1}{1.2}$+$\frac{1}{2.3}$+$\frac{1}{3.4}$+$\frac{1}{4.5}$+$\frac{1}{5.6}$
=1-$\dfrac{1}{2}$+$\dfrac{1}{2}$-$\dfrac{1}{3}$+$\dfrac{1}{3}$-$\dfrac{1}{4}$+$\dfrac{1}{4}$- $\dfrac{1}{5}$+$\dfrac{1}{5}$-$\dfrac{1}{6}$
=1-$\dfrac{1}{6}$
=$\dfrac{5}{6}$
