1: Ta có:
1/2^2<1/1.2
1/3^2<1/2.3
.......
1/45^2<1/44.45
Suy ra:1/2^2+1/3^2+1/4^2+...+1/45^2<1/1.2+1/2.3+1/3.4+...+1/44.45
Mà:1/1.2+1/2.3+1/3.4+...+1/44.45
=1-1/2+1/2-1/3+1/3-1/4+...+1/44-1/45
=1-1/45
=44/45<1
Mà:1/2^2+1/3^2+1/4^2+...+1/45^2<1/1.2+1/2.3+1/3.4+...+1/44.45
Suy ra:1/2^2+1/3^2+1/4^2+...+1/45^2<44/45<1
Vậy:1/2^2+1/3^2+1/4^2+...+1/45^2<1
2:Ta có:
1/5+1/6+1/7+...+1/17<1/5+1/6+1/7.11
Suy ra: 1/5+1/6+1/7+...+1/17<407/210<2
Vậy: 1/5+1/6+1/7+...+1/17<2.
Không cần cảm ơn đâu nhé, mình biết mình giỏi rồi!hì hì