@Active Activity.
$A=$ `1/1×2` $+$ `1/2×3` $+$ `1/3×4` $+$...$+$ `1/199×200`
$A=$ `1/1` $-$ `1/2` $+$ `1/2` $-$ `1/3` $+$ `1/3` $-$ `1/4` $+$...$+$ `1/199` $-$ `1/200`
$A=$ `1/1` $-$ `1/200`
$A=$ `199/200`
$#Nocopy.$
$\text{Chúc bạn học tốt, xin hay nhất nhé !!!}$