$\frac{1}{1.2}$ + $\frac{1}{2.3}$ + $\frac{1}{3.4}$ +...+ $\frac{1}{19.20}$ = x - $\frac{1}{20}$
<=> `1`-$\frac{1}{2}$ + $\frac{1}{2}$ -$\frac{1}{3}$ + ... + $\frac{1}{18}$ - $\frac{1}{19}$ + $\frac{1}{19}$ - $\frac{1}{20}$ = x - $\frac{1}{20}$
<=> `1` - $\frac{1}{20}$ = x - $\frac{1}{20}$
<=> x - $\frac{1}{20}$ = `1` - $\frac{1}{20}$
<=> x = `1` - $\frac{1}{20}$ + $\frac{1}{20}$
<=> x = `1`
Chúc bạn học tốt !!!!!
