Tính hợp lí
A = 4/4.7 + 4/7.10 + ...... + 4/79.82
Những câu hỏi liên quan
Tính:
3/1.4 + 4/4.7 +3/7.10 + ...+ 3/40.43
Sửa đề : \(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{40.43}\)
\(=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{40}-\frac{1}{43}\)
\(=1-\frac{1}{43}=\frac{42}{43}\)
6. Tính
\(A=\dfrac{4}{1.4}+\dfrac{4}{4.7}+\dfrac{4}{7.10}+...+\dfrac{4}{31.34}\)
\(B=1-5+5^2-5^3+5^4-...-5^{39}\)
a) Ta có: \(A=\dfrac{4}{1\cdot4}+\dfrac{4}{4\cdot7}+\dfrac{4}{7\cdot10}+...+\dfrac{4}{31\cdot34}\)
\(=\dfrac{4}{3}\left(\dfrac{3}{1\cdot4}+\dfrac{3}{4\cdot7}+\dfrac{3}{7\cdot10}+...+\dfrac{3}{31\cdot34}\right)\)
\(=\dfrac{4}{3}\left(1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+...+\dfrac{1}{31}-\dfrac{1}{34}\right)\)
\(=\dfrac{4}{3}\left(1-\dfrac{1}{34}\right)\)
\(=\dfrac{4}{3}\cdot\dfrac{33}{34}=\dfrac{22}{17}\)
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C = 4/4.7 + 4/7.10 + 4/10.13 +....+ 4/73.76
a = 2/1.4+2/4.7 +4/7.10 +... + 2/97.100
=2/3(3/1*4+3/4*7+...+3/97*100)
=2/3(1-1/4+1/4-1/7+...+1/97-1/100)
=2/3*99/100
=198/300
=66/100
=33/50
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Tính: D=3/4+25/4.7+67/7.10+...+10297/100.103
giúp mk vs mk cần gấp
\(\frac{4}{1.4}+\frac{4}{4.7}+\frac{4}{7.10}+\frac{4}{10.13}\)
4/1.4+4/4.7+4/7.10+4/10.13
= 4/3(3/1.4+3/4.7+3/7.10+3/10.13)
=4/3(1/1-1/4+1/4-1/7+1/7-1/10+1/10-1/13)
=4/3(1/1-1/13)
=4/3.12/13
=16/13
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1/1*4+1/4.7+1/7.10+.....+1/94.97+1/97.101
Tính số hạng 50 của các dãy sau
a, 1. 6 ; 2 . 7; 3 . 8;...
b, 1 . 4; 4.7 ; 7.10;...
so sánh M và N: M=4/4.7+4/7.10+4/10.13+....+4/87.90 ; N=10^8+1/10^8-1