tính nhanh 5^2/5x6+5^2/6x11+5^2/16x21+5^2./21x26+5^2/26x31+5^2/31x36 giúp mk nha. mk tích cho
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làm giúp mình nha .tính nhanh 5^2/5x6+5^2/6x11+5^2/16x21+5^2./21x26+5^2/26x31+5^2/31x36
5^2/1x6+5^2/6x11+5^2/11x16+5^/16x21+5^2/21x26+5^2/26x31
tinh hop ly
tính tổng :
S = (5^2 / 1x6) + (5^2 / 6x11) +( 5^2 / 11x16) + (5^2 / 16x21) + (5^2 / 21x26)
5^2/1x6+5^2/6x11+5^2/11x16+5^2/16x21+5^2/21x26
\(\frac{5^2}{1.6}+\frac{5^2}{6.11}+\frac{5^2}{11.16}+\frac{5^2}{16.21}+\frac{5^2}{21.26}\)
\(=5^2\left(\frac{1}{1.6}+\frac{1}{6.11}+\frac{1}{11.16}+\frac{1}{16.21}+\frac{1}{21.26}\right)\)
\(=25.\frac{1}{5}\left(1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+...+\frac{1}{21}-\frac{1}{26}\right)\)
\(=5\left(1-\frac{1}{26}\right)\)
\(=5.\frac{25}{26}\)
\(=\frac{125}{26}\)
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5^2/1x6+5^2/6x11+5^2/11x16+5^2/16x21+5^2/21x26
s= 5^2 / 1x 6+ 5^2 / 6x11 +5^2/11x16+5^2/16x21+5^2/ 21x26
\(S=\frac{5^2}{1.6}+\frac{5^2}{6.11}+\frac{5^2}{11.16}+\frac{5^2}{16.21}+\frac{5^2}{21.26}\)
\(5S=5^2\left(\frac{1}{1.6}+\frac{1}{6.11}+\frac{1}{11.16}+\frac{1}{16.21}+\frac{1}{21.26}\right)\)
\(5S=5^2\left(1-\frac{1}{26}\right)\)
\(\Rightarrow S=5^2:5\left(1-\frac{1}{26}\right)\)
\(S=5\left(1-\frac{1}{26}\right)\)
\(S=5.\frac{25}{26}\)
\(S=\frac{125}{26}\)
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\(S=\frac{5^2}{1x6}+\frac{5^2}{6x11}+\frac{5^2}{11x16}+\frac{5^2}{16x21}+\frac{5^2}{21x26}\)
\(S=\frac{5^2}{1\cdot6}+\frac{5^2}{6\cdot11}+\frac{5^2}{11\cdot16}+\frac{5^2}{16\cdot21}+\frac{5^2}{21\cdot26}\)
\(S=\frac{25}{1\cdot6}+\frac{25}{6\cdot11}+\frac{25}{11\cdot16}+\frac{25}{16\cdot21}+\frac{25}{21\cdot26}\)
\(S=5\left[\frac{5}{1\cdot6}+\frac{5}{6\cdot11}+\frac{5}{11\cdot16}+\frac{5}{16\cdot21}+\frac{5}{21\cdot26}\right]\)
\(S=5\left[1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+...+\frac{1}{21}-\frac{1}{26}\right]\)
\(S=5\left[1-\frac{1}{26}\right]=5\cdot\frac{25}{26}=\frac{125}{26}\)
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Bài làm
S = \(\frac{5^2}{1.6}\)+ \(\frac{5^2}{6.11}\)+ \(\frac{5^2}{11.16}\)+ \(\frac{5^2}{16.21}\)+\(\frac{5^2}{21.26}\)
S : 5 = \(\frac{5}{1.6}\)+ \(\frac{5}{6.11}\)+ \(\frac{5}{11.16}\) + \(\frac{5}{16.21}\) + \(\frac{5}{21.26}\)
S : 5 = 1 - \(\frac{1}{6}\)+ \(\frac{1}{6}\)- \(\frac{1}{11}\) + \(\frac{1}{11}\)- \(\frac{1}{16}\)+ \(\frac{1}{16}\)- \(\frac{1}{21}\)+ \(\frac{1}{21}\)- \(\frac{1}{26}\)
S : 5 = 1 - \(\frac{1}{26}\)
S : 5 = \(\frac{25}{26}\)
S = \(\frac{125}{26}\)
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\(S=\frac{5^2}{1.6}+\frac{5^2}{6.11}+\frac{5^2}{11.16}+\frac{5^2}{16.21}+\frac{5^2}{21.26}\)
\(S=5.\left(\frac{5}{1.6}+\frac{5}{6.11}+\frac{5}{11.16}+\frac{5}{16.21}+\frac{5}{21.26}\right)\)
\(S=5\left(\frac{1}{1}-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+\frac{1}{21}-\frac{1}{26}\right)\)
\(S=5\left(1-\frac{1}{26}\right)\)
\(S=5.\frac{25}{26}\)
=>\(S=\frac{125}{26}\)
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\(A=\frac{5}{11x16}+\frac{5}{16x21}+\frac{5}{21x26}+....+\frac{5}{61x66}\)
\(C=\frac{2}{3x4}+\frac{2}{4x5}+....+\frac{2}{40x41}+\frac{2}{41x42}\)
Ai nhanh mk tik nha!!!
Ta có : \(C=\frac{2}{3.4}+\frac{2}{4.5}+\frac{2}{5.6}+......+\frac{2}{41.42}\)
\(C=2\left(\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+.....+\frac{1}{41.42}\right)\)
\(C=2\left(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+.....+\frac{1}{41}-\frac{1}{42}\right)\)
\(C=2\left(\frac{1}{3}-\frac{1}{42}\right)\)
\(C=2.\frac{13}{42}=\frac{13}{21}\)
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Các bạn chỉ cần trả lời câu C thôi nha!!!
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Ta có : \(A=\frac{5}{11.16}+\frac{5}{16.21}+\frac{5}{21.26}+......+\frac{5}{61.66}\)
\(\Rightarrow A=\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+.......+\frac{1}{61}-\frac{1}{66}\)
\(\Rightarrow A=\frac{1}{11}-\frac{1}{66}\)
\(\Rightarrow A=\frac{5}{66}\)
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A= 55/11x16 + 55/16x21 + 55/21x26 + 55/26x31 + 55/31x36 + 55/36x41
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