Ta có:
a=1/2x2+1/3x4+1/5x6+...+1/99x100
a=1-1/2+1/3-1/4+1/5-1/6+...+1/99-1/100
a=1+1/2+1/3+1/4+1/5+1/6+...+1/99+1/100-2.1/2-2.1/4-...-2.1/98
a=1+... +1/100-1-1/2-1/3-...-1/49
a=1/51+...+1/100
=> a<1/51.25=25/51<25/30=5/6=>đpcm
Bài2:
và: a>25x1/75+25x1/100=7/12
NHỚ CHO MK TRẢ LỜI HAY NHẤT NHÉ!!!!!!