Giải thích các bước giải:
$\text{$\frac{-1}{1×2}$+$\frac{-1}{2×3}$+$\frac{-1}{3×4}$+$\frac{-1}{4×5}$}$
$\text{=-1×($\frac{1}{1×2}$+$\frac{1}{2×3}$+$\frac{1}{3×4}$+$\frac{1}{4×5}$)}$
$\text{=-1×(1-$\frac{1}{2}$+$\frac{1}{2}$-$\frac{1}{3}$+...+$\frac{1}{4}$-$\frac{1}{5}$)}$
$\text{=-1×[1-($\frac{1}{2}$-$\frac{1}{2}$)-($\frac{1}{3}$-$\frac{1}{3}$)-...-$\frac{1}{5}$]}$
$\text{=-1×[1-$\frac{1}{5}$]}$
$\text{=-1×$\frac{4}{5}$}$
$\text{=$\frac{-4}{5}$}$
