Đáp án:
C)
Ta có : 1/2^2=1/4
1/3^2<1/(2.3)
....
1/100^2<1/(99.100)
⇒ 1/2^2 + 1/3^2 + .... + 1/100^2 < 1/4 + 1/(2.3) + .... + 1/(99.100)
< 1/4 + 1/2 - 1/3 + ....+ 1/99 - 1/100
< 1/4 + 1/2 - 1/100
< 3/4 - 1/100 < 3/4
d) D = 1/2^2 . ( 1 + 1/2^2 + 1/3^2 + ... + 1/50^2 )
< 1/4 . ( 1 + 1/(1.2) + 1/(2.3) + .... + 1/(49.50) )
= 1/4 ( 1 + 1 - 1/2 + 1/2 - 1/3 + ... + 1/49 - 1/50 )
= 1/4 . ( 1 + 1 - 1/50 ) = 99/200 < 1/2
Mình chỉ biết làm Ý c thôi
