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To solve the equations: {5x-4y = 12} {3x-y = 7}
5x-4y=12 ①
3x-y=7 ②
② It's 4
12x-4y=28 ③
③ - 1. It's necessary
7x=16
x=16/7
Substituting in 2, we get
y=-1/7
The solution of the original equations is x = 16 / 7, y = - 1 / 7
First observe the following formulas: 1 / 1 * 4 = 1 / 3 * (1-1 / 4), 1 / 4 * 7 = 1 / 3 * (1 / 4-1 / 7), 1 / 7 * 10 = 1 / 3 * (1 / 7-18) ,
First observe the following formulas: 1 / 1 * 4 = 1 / 3 * (1-1 / 4), 1 / 4 * 7 = 1 / 3 * (1 / 4-1 / 7), 1 / 7 * 10 = 1 / 3 * (1 / 7-18) According to the above rule, we can calculate the following formula: 1 / 1 * 4 + 1 / 4 * 7 + 1 / 7 * 10 + +1, 2008 * 2001
1 * 4 is one times one quarter, and * is the multiplication sign
1/1*4+1/4*7+1/7*10+...+1/2005*2008+1/2008*2011
=1/3（1-1/4)+1/3（1/4-1/7)+1/3(1/7-1/10)+.+1/3(1/2005-1/2008)+1/3(1/2008-1/2011)
=1/3（1-1/4+1/4-1/7+1/7-1/10+.+1/2005-1/2008+1/2008-1/2011)
=1/3(1-1/2011)
=1/3(2010/2011)
=670/2011
1/1X4=1/3(1-1/4) 1/4X7=1/3(1/4-1/7) 1/7x10=1/3(1/7-1/10) 1/N(N+3)=1/3(1/N-1/N+3)
Xiao Ming read a book. In the morning, he read part of it. At this time, the ratio of the number of pages read to the number of pages not read is 1:9. In the afternoon, he read six more pages than in the morning. At this time, the ratio of the number of pages read to the number of pages not read becomes 1:3?
A: this book has 120 pages
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