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Lời giải:
$12A=1.5.12+5.9.(13-1)+9.13(17-5)+13.17(21-9)+....+77.81(85-73)+81.85(89-77)$
$=60+(5.9.13+9.13.17+13.17.21+...+77.81.85+81.85.89)-(1.5.9+5.9.13+9.13.17+...+73.77.81+77.81.85)$
$=60+81.85.89 - 1.5.9=612780$
A = 1.5 + 5.9 + 9.13 + ... + 81.85
A = \(\dfrac{12}{12}\)(1.5 + 5.9 + 9.13 + 81.85)
A = \(\dfrac{1}{12}\).(1.5.12 + 5.9.12.+ 9.13.12 + ...+ 81.85.12]
A = \(\dfrac{1}{12}\).[1.5.(9 + 3) + 5.9.(13 - 1) + 9.13.(17 - 5) +...+ 81.85.(89 - 77)]
A = \(\dfrac{1}{12}\).[1.5.9 + 1.3.5 + 5.9.13 - 5.9.1 + 9.13.17 - 9.13.5 + ...+ 81.85.89 - 81.85.77]
A = \(\dfrac{1}{12}\).[1.3.5 + 81.85.89]
A = \(\dfrac{1}{12}\).[15 + 612765]
A = \(\dfrac{1}{12}\).612780
A = 51065
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\(E=\dfrac{3}{1x5}+\dfrac{3}{5x9}+...+\dfrac{3}{121x125}\)
\(\dfrac{4}{3}xE=1-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{9}+...+\dfrac{1}{121}-\dfrac{1}{125}\)
\(\dfrac{4}{3}xE=1-\dfrac{1}{125}\)
\(E=\dfrac{124}{125}x\dfrac{3}{4}=\dfrac{93}{125}\)
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7/1x5+7/5x9+7/9x13+7/13x17+7/17x21=75,5 120 1609
ஜ۩۞۩ஜ Ủng hộ tui nhaaaa ஜ۩۞۩ஜ
Ai đi qua xin để lại 1 k !!!!!!
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\(A=\frac{2}{1.5}+\frac{2}{5.9}+\frac{2}{9.13}+....+\frac{2}{81.85}\)
\(\Rightarrow2A=\frac{4}{1.5}+\frac{4}{5.9}+\frac{4}{9.13}+....+\frac{4}{81.85}\)
\(\Rightarrow2A=1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+....+\frac{1}{81}-\frac{1}{85}\)
\(\Rightarrow2A=1-\frac{1}{85}\)
\(\Rightarrow A=\frac{84}{85}:2=\frac{42}{85}\)
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
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a) \(\frac{1}{1.3}+\frac{1}{3.5}+....+\frac{1}{47.49}=\frac{24}{x+4}\)
\(\Rightarrow\frac{1}{2}.\left(\frac{2}{1.3}+\frac{2}{3.5}+......+\frac{2}{47.49}\right)=\frac{24}{x+4}\)
\(\Rightarrow\frac{1}{2}.\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+.....+\frac{1}{47}-\frac{1}{49}\right)=\frac{24}{x+4}\)
\(\Rightarrow\frac{1}{2}.\left(1-\frac{1}{49}\right)=\frac{24}{x+4}\)
\(\Rightarrow\frac{1}{2}.\frac{48}{49}=\frac{24}{x+4}\)
\(\Rightarrow\frac{24}{49}=\frac{24}{x+4}\)
\(\Rightarrow x+4=49\Rightarrow x=45\)
\(A=\frac{1}{1.3}+\frac{1}{3.5}+...+\frac{1}{47.49}=\frac{24}{x+4}\)
\(\Rightarrow A=\frac{1}{2}.\left(\frac{2}{1.3}+\frac{2}{3.5}+...+\frac{2}{47.49}\right)=\frac{24}{x+4}\)
\(\Rightarrow A=\frac{1}{2}.\left(\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{47}-\frac{1}{49}\right)=\frac{24}{x+4}\)
\(\Rightarrow A=\frac{1}{2}.\left(\frac{1}{1}-\frac{1}{49}\right)=\frac{24}{x+4}\Leftrightarrow A=\frac{1}{2}.\frac{48}{49}=\frac{24}{x+4}\)
\(\Rightarrow A=\frac{24}{49}=\frac{24}{x+4}\Leftrightarrow x=49-4=45\)
Bài b) hình như sai đề thì phải đó bạn.
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a) 1/1x5 + ... + 1/21x25
= 4 x (1-1/5 + 1/5 - 1/9 + ... + 1/21 - 1/25)
= 1/4 x (1 - 1/25)
= 1/4 x 24/25
= 6/25
làm bừa thui,ai tích mình mình tích lại
Số số hạng là :
Có số cặp là :
50 : 2 = 25 ( cặp )
Mỗi cặp có giá trị là :
99 - 97 = 2
Tổng dãy trên là :
25 x 2 = 50
Đáp số : 50
A = 1.5 + 5.9 + 9.13 + ...+ 41.45
A = \(\dfrac{12}{12}\).(1.5 + 5.9 + 9.13 +...+ 41.45)
A = \(\dfrac{1}{12}\).(1.5.12 + 5.9.12 + 9.13.12 + ... + 41.45.12)
A = \(\dfrac{1}{12}\).[1.5.(9 + 3) + 5.9.(13 - 1) + 9.13.(17 - 5) + ... + 41.45.(49 - 37)]
A = \(\dfrac{1}{12}\)[1.5.9 + 1.5.3 + 5.9.13 - 1.5.9 + 9.13.17 - 5.9.13 +...+41.45.49 - 41.45.37]
A =\(\dfrac{1}{12}\).[1.5.3 + 41.45.49]
A = \(\dfrac{1}{12}\).[15 + 90405]
A = \(\dfrac{1}{12}\).90420
A = 7535
.