tớ nghĩ là A = B 

nếu đúng thì tk hộ tớ với , tớ đang bị âm 998 điểm !

dễ thôi nhưng tôi giúp câu A còn đâu bn tự nghĩ nhé

\(A=\frac{2}{1.5}+\frac{2}{5.9}+\frac{2}{9.13}+....+\frac{2}{81.85}\)

\(\Rightarrow2A=\frac{4}{1.5}+\frac{4}{5.9}+\frac{4}{9.13}+....+\frac{4}{81.85}\)

\(\Rightarrow2A=1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+....+\frac{1}{81}-\frac{1}{85}\)

\(\Rightarrow2A=1-\frac{1}{85}\)

\(\Rightarrow A=\frac{84}{85}:2=\frac{42}{85}\)

tính A còn lại tự tính nha

a) A = 2/1x5 + 2/5x9 + 2/9x13 +....+2/81x85

\(\frac{2}{1x5}+\frac{2}{5x9}+\frac{2}{9x13}+...+\frac{2}{81x85}\)

\(A=1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+...+\frac{1}{81}-\frac{1}{85}\)

\(A=1-\frac{1}{85}\)

\(\Rightarrow A=\frac{84}{85}\)

k nha

Sửa đề: \(G=\frac{3}{4\times9}+\frac{3}{9\times14}+\frac{3}{14\times19}+\cdots+\frac{3}{34\times39}\)

Ta có: \(G=\frac{3}{4\times9}+\frac{3}{9\times14}+\frac{3}{14\times19}+\cdots+\frac{3}{34\times39}\)

\(=\frac35\times\left(\frac{5}{4\times9}+\frac{5}{9\times14}+\cdots+\frac{5}{34\times39}\right)\)

\(=\frac35\times\left(\frac14-\frac19+\frac19-\frac{1}{14}+\cdots+\frac{1}{34}-\frac{1}{39}\right)\)

\(=\frac35\times\left(\frac14-\frac{1}{39}\right)=\frac35\times\frac{35}{156}=\frac{7}{52}\)

Đặt tổng trên là A

\(5A=\frac{5}{4x9}+\frac{5}{9x14}+\frac{5}{14x19}+...+\frac{5}{44x49}\)

\(5A=\frac{9-4}{4x9}+\frac{14-9}{9x14}+\frac{19-14}{14x19}+...+\frac{49-44}{44x49}\)

\(5A=\frac{1}{4}-\frac{1}{9}+\frac{1}{9}-\frac{1}{14}+\frac{1}{14}-\frac{1}{19}+...+\frac{1}{44}-\frac{1}{49}\)

\(5A=\frac{1}{4}-\frac{1}{49}\Rightarrow A=\frac{49-4}{4x5x49}=\frac{45}{4x5x49}=\frac{9}{4x49}\)

Đề bài?

Đúng jui đó bn Hạ Ninh !!!

Đặt \(A=\frac{3}{1\cdot4}+\frac{5}{4\cdot9}+\frac{7}{9\cdot16}+\frac{9}{16\cdot25}+\frac{11}{25\cdot36}\)

\(A=\frac{4-1}{1\cdot4}+\frac{9-4}{4\cdot9}+\frac{16-9}{9\cdot16}+\frac{25-16}{16\cdot25}+\frac{36-25}{25\cdot36}\)

\(A=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{9}+\frac{1}{9}-\frac{1}{16}+\frac{1}{16}-\frac{1}{25}+\frac{1}{25}-\frac{1}{36}\)

\(\Rightarrow A=1-\frac{1}{36}=\frac{35}{36}\)

= 1-\(\frac{1}{4}\)+ \(\frac{1}{4}\)-\(\frac{1}{9}\)+\(\frac{1}{9}\)-\(\frac{1}{16}\)+\(\frac{1}{16}\)-\(\frac{1}{25}\)+\(\frac{1}{25}\)-\(\frac{1}{36}\)=1-\(\frac{1}{36}\)=35/36

2 x 2 x 3 = 12

40 : 4 : 5= 2

4 x 9 + 6= 42

2 x 8 + 72 = 88

2x2x3=12

40:4:5=2

4x9+6=42

2x8+72=88