Xét số hạng tổng quát thứ n (n nguyên và n>1), ta có 
1/n(1+2+...+n)=[n(n+1)/2]/n= [n(n+1)]/(2n) 
Do đó 
B = 1 + 1/2 (1 + 2) + 1/3 (1 + 2 + 3) + 1/4 (1 + 2 + 3 +4) + ...+ 1/20 (1 + 2 +... + 20) 
=1 +[2(2+1)]/(2.2) +[3(3+1)]/(2.3) +[4(4+1)]/(2.4) +... +[20(20+1)]/(2.20) 
=1+3/2 +4/2 +5/2 +... +21/2 
=(2+3+4+5+...+20)/2=104,5 . TICH CHON MINH NHA CAC BAN THI CA NAM SE GAP NHIEU DIEU MAY MAN DAY

Xét số hạng tổng quát thứ n (n nguyên và n>1), ta có 
1/n(1+2+...+n)=[n(n+1)/2]/n= [n(n+1)]/(2n) 
Do đó 
B = 1 + 1/2 (1 + 2) + 1/3 (1 + 2 + 3) + 1/4 (1 + 2 + 3 +4) + ...+ 1/20 (1 + 2 +... + 20) 
=1 +[2(2+1)]/(2.2) +[3(3+1)]/(2.3) +[4(4+1)]/(2.4) +... +[20(20+1)]/(2.20) 
=1+3/2 +4/2 +5/2 +... +21/2 
=(2+3+4+5+...+20)/2=104,5 

Xét số hạng tổng quát thứ n (n nguyên và n>1), ta có 
1/n(1+2+...+n)=[n(n+1)/2]/n= [n(n+1)]/(2n) 
Do đó 
B = 1 + 1/2 (1 + 2) + 1/3 (1 + 2 + 3) + 1/4 (1 + 2 + 3 +4) + ...+ 1/20 (1 + 2 +... + 20) 
=1 +[2(2+1)]/(2.2) +[3(3+1)]/(2.3) +[4(4+1)]/(2.4) +... +[20(20+1)]/(2.20) 
=1+3/2 +4/2 +5/2 +... +21/2 
=(2+3+4+5+...+20)/2=104,5 

B = 1+[1/2 (1+2) 2]/2 +[1/3 (1+2+3) 3]/2 +....+ [1/16 (1+2+3+...+20) 16] /2

B = 1+3/2 + 4/2 +...+ 17/2

B = 1/2 (2+3+4+....+17)

B= 1/2 [(2+7)16]/2

B= 76

Nhớ k cho mình nhé :D

Viết lại đề bài

\(B=1+\frac{1}{2\left(1+2\right)}+\frac{1}{3\left(1+2+3\right)}+\frac{1}{4\left(1+2+3+4\right)}+...+\frac{1}{20\left(1+2+3+4...+20\right)}\)

12(1+2)+13(1+2+3)+...+120(1+2+...+20)" role="presentation" style="border:0px; box-sizing:border-box; direction:ltr; display:inline-block; float:none; font-size:15.82px; line-height:0; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; overflow-wrap:normal; padding:1px 0px; position:relative; white-space:nowrap; word-break:break-word; word-spacing:normal" class="MathJax_CHTML mjx-chtml">12(1+2)+13(1+2+3)+...+120(1+2+...+20)

12.2.3:2+13.3.4:2+...+120.20.21:2" role="presentation" style="border:0px; box-sizing:border-box; direction:ltr; display:inline-table; float:none; font-size:15.82px; line-height:0; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; overflow-wrap:normal; padding:1px 0px; position:relative; white-space:nowrap; word-break:break-word; word-spacing:normal" class="MathJax_CHTML mjx-chtml">12.2.3:2+13.3.4:2+...+120.20.21:2

22+32+...+212" role="presentation" style="border:0px; box-sizing:border-box; direction:ltr; display:inline-block; float:none; font-size:15.82px; line-height:0; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; overflow-wrap:normal; padding:1px 0px; position:relative; white-space:nowrap; word-break:break-word; word-spacing:normal" class="MathJax_CHTML mjx-chtml">22+32+...+212

2+3+...+212" role="presentation" style="border:0px; box-sizing:border-box; direction:ltr; display:inline-block; float:none; font-size:15.82px; line-height:0; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; overflow-wrap:normal; padding:1px 0px; position:relative; white-space:nowrap; word-break:break-word; word-spacing:normal" class="MathJax_CHTML mjx-chtml">2+3+...+212

2302" role="presentation" style="border:0px; box-sizing:border-box; direction:ltr; display:inline-block; float:none; font-size:15.82px; line-height:0; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; overflow-wrap:normal; padding:1px 0px; position:relative; white-space:nowrap; word-break:break-word; word-spacing:normal" class="MathJax_CHTML mjx-chtml">2302

$\Rightarrow B=115$