[3 câu tiếp theo nè!]
4) D = \(\dfrac{1}{25.27}\) + \(\dfrac{1}{27.29}\) + ... + \(\dfrac{1}{73.75}\)
5) E = \(\dfrac{15}{90.94}\) + \(\dfrac{15}{94.98}\) + ... + \(\dfrac{15}{146.150}\)
6) F = \(\dfrac{10}{56}\) + \(\dfrac{10}{140}\) + \(\dfrac{10}{260}\) + ... + \(\dfrac{10}{1400}\)
Các bạn cố gắng giải đầy đủ giúp mình!
D= 1/2. (1/25-1/27 +1/27-1/29+...+1/73-1/75)
= 1/2. (1/25 -1/75)
=1/2 . 2/75= 1/75
D = \(\dfrac{1}{25.27}+\dfrac{1}{27.29}+...+\dfrac{1}{73.75}\)
2D = 2( \(\dfrac{1}{25.27}+\dfrac{1}{27.29}+...+\dfrac{1}{73.75}\) )
= \(\dfrac{2}{25.27}+\dfrac{2}{27.29}+...+\dfrac{2}{73.75}\)
= \(\dfrac{1}{25}-\dfrac{1}{27}+\dfrac{1}{27}-\dfrac{1}{29}+...+\dfrac{1}{73}-\dfrac{1}{75}\)
= \(\dfrac{1}{25}-\dfrac{1}{75}\)
= \(\dfrac{2}{75}\)
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5) \(E=\dfrac{15}{90.94}+\dfrac{15}{94.98}+...+\dfrac{15}{146.150}\)
\(\dfrac{4E}{15}=\dfrac{4}{90.94}+\dfrac{4}{94.98}+...+\dfrac{4}{146.150}\)
\(\dfrac{4E}{15}=\dfrac{1}{90}-\dfrac{1}{94}+\dfrac{1}{94}-\dfrac{1}{98}+...+\dfrac{1}{146}-\dfrac{1}{150}\)
\(\dfrac{4E}{15}=\dfrac{1}{90}-\dfrac{1}{150}=\dfrac{1}{225}\)
\(4E=\dfrac{1}{15}\)
\(E=\dfrac{1}{60}\)
Bài 6 ) bạn làm tương tự như bài 5) nha
