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E=10/56 + 10/140 + 10/260 +...+10/1400
E=5/28+5/70+5/130+...+5/700
3E/5=3/4.7+3/7.10+3/10.13+...+3/25.28
3E/5=1/4-1/7+1/7-1/10+1/10-1/13+...+1/25-1/28
3E/5=1/4-1/28
3E/5=3/14
E=3/14.5/3
E=5/14
Vậy E=5/14
10/56+10/140+10/260 +....+10/1400
=5/28+5/70+5/130+...+5/700
=5/4.7+5/7.10+5/10.13+...+5/25.28
=5/3(1/4-1/7+1/7-1/10+1/10-1/13+.....+1/25-1/28)
=5/3(1/4-1/28)
=5/3.3/14
=5/14
Đầu tiên rút gọn S trước
S= 5/28 + 5/70 +.....+10/700
= 5/(4.7)+5/(7.10)+....5/(25.28)
3S= 5( 1/4 - 1/7 +1/7-1/10+......+1/25-1/28)
3S= 5 (1/4-1/28)
3S=15/14
S= 5/14
tích nha
B=5/28+5/70+5/130+...+5/700
B=5/4.7+5/7.10+5/10.13+...+5/25.28
B=5/3.(3/4.7+3/7.10+3/10.13+...+3/25.28)
B=5/3.(1/4-/7+1/7-1/10+1/10+1/10-1/13+...+1/25-1/28)
B=5/3.(1/4-1/28)
B=5/3.3/14
B=5/14
Vậy B=5/14
B=5/3.(3/28+3/70+...+3/700)
B=5/3.(3/4.7+3/7.10+...+3/25.28)
B=5/3.(1/4-1/7+1/7-1/10+...+1/25-1/28)
B=5/3.(1/4-1/28)
B=5/3.3/14
B=5/14
Ko bít có đúng ko, cứ thử đi nhé
Đầu tiên , ta rút gọn C :
C = 5/28 + 5/70 +.....+10/700
C = 5/(4.7)+5/(7.10)+....5/(25.28)
3C= 5( 1/4 - 1/7 +1/7-1/10+......+1/25-1/28)
3C= 5 (1/4-1/28)
3C=15/14
C= 5/14
\(\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}\)
\(=\frac{5}{28}+\frac{5}{70}+\frac{5}{130}+...+\frac{5}{700}\)
\(=\frac{5}{4.7}+\frac{5}{7.10}+\frac{5}{10.13}+...+\frac{5}{25.28}\)
\(=\frac{5}{3}\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{25}-\frac{1}{28}\right)\)
\(=\frac{5}{3}\left(\frac{1}{4}-\frac{1}{28}\right)=\frac{5}{3}.\frac{3}{14}=\frac{5}{14}\)
ths bn