Đáp án:
Giải thích các bước giải:
a.
B = $\frac{196 + 197}{197 + 198}$ = $\frac{196}{197 + 198}$ + $\frac{197}{197 + 198}$
<$\frac{197}{198}$ + $\frac{196}{197}$ = A
⇒ A > B
b.
B = $\frac{1}{6}$ + $\frac{1}{12}$ + $\frac{1}{20}$ + $\frac{1}{30}$ + $\frac{1}{42}$ + $\frac{1}{56}$ + $\frac{1}{72}$ + $\frac{1}{90}$
= $\frac{1}{2.3}$ + $\frac{1}{3.4}$ + $\frac{1}{4.5}$ + $\frac{1}{5.6}$ + $\frac{1}{6.7}$ + $\frac{1}{7.8}$ + $\frac{1}{8.9}$ + $\frac{1}{9.10}$
= $\frac{1}{2}$ - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{4}$ + $\frac{1}{4}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{6}$ + $\frac{1}{6}$ - $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{8}$ + $\frac{1}{8}$ - $\frac{1}{9}$ + $\frac{1}{9}$ - $\frac{1}{10}$
= $\frac{1}{2}$ - $\frac{1}{10}$
= $\frac{5}{10}$ - $\frac{1}{10}$
= $\frac{4}{10}$
= $\frac{2}{5}$
