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![](https://rs.olm.vn/images/avt/0.png?1311)
3/4+1/4:x=-3
1/4:x=(-3)-3/4
1/4:x=-15/4
x=-15/4.1/4
x=-15/16
đúng nha bn
![](https://rs.olm.vn/images/avt/0.png?1311)
1 + 5 = 6
ai k mik đi
Hứa là ai k lại mỗi ngày bạn ấy sẽ có 1 k.
Nhớ nhắn tin cho mik để mik biết
mà k nhé.
thanks.
![](https://rs.olm.vn/images/avt/0.png?1311)
\(Xét:1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+...-\frac{1}{2012}\)
\(=\left(1+\frac{1}{3}+...+\frac{1}{2011}\right)-\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{2012}\right)\)
\(=\left(1+\frac{1}{3}+...+\frac{1}{2011}\right)\)\(+\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{2012}\right)\)\(-\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{2012}\right)\)\(+\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{2012}\right)\)
\(=\left(1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{2011}+\frac{1}{2012}\right)\)\(-2\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{2012}\right)\)
\(=\left(1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{2011}+\frac{1}{2012}\right)\)\(-\left(1+\frac{1}{2}+...+\frac{1}{1006}\right)\)
\(=\frac{1}{1007}+\frac{1}{1008}+...+\frac{1}{2012}\)
\(\RightarrowĐPCM\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Bài 1 :
\(a,\left(a-b\right)+\left(c-d\right)-\left(a-c\right)=-\left(b+d\right)\)
Ta có : \(VT=\left(a-b\right)+\left(c-d\right)-\left(a-c\right)\)
\(=a-b+c-d-a+c\)
\(=-\left(b+d\right)=VP\)
\(\Rightarrow\left(a-b\right)+\left(c-d\right)-\left(a-c\right)=-\left(b+d\right)\)
\(b,\left(a-b\right)-\left(c-d\right)+\left(b+c\right)=a+d\)
Ta có : \(VT=\left(a-b\right)-\left(c-d\right)+\left(b+c\right)\)
\(=a-b-c+d+b+c\)
\(=a+d=VP\)
\(\Rightarrow\left(a-b\right)-\left(c-d\right)+\left(b+c\right)=a+d\)
![](https://rs.olm.vn/images/avt/0.png?1311)
ta thấy (x+1)(2y-5)=143=11.13=13.11=143.1=1.143
Suy ra ta có 4 trường hợp sau:
Nếu x + 1 = 11 suy ra ta x = 10 ; 2y - 5 = 13 suy ra y = 9
Nếu y + 1 = 13 suy ra y = 12 ; 2y-5=11 suy ra y = 8
Nếu x + 1 = 143 suy ra x = 142 ; 2y - 5 = 1 suy ra y = 3
Nếu x + 1 = 1 suy ra x = 0 ; 2y - 5 = 143 suy ra y = 74
Vậy x = 10 thì y = 9
x = 12 thì y = 8
x = 142 thì y = 3
x = 0 thì y = 74
Vì (x+1)(2y-5) = 143 => x+1;2y-5 \(\in\)Ư(143) = {1;11;13;143}
Ta có:
Nếu x+1=1;2y-5=143=> x=0;y=74
Nếu x+1=11;2y-5=13=>x=10;y=9
Nếu x+1=13;2y-5=11=>x=12;y=8
Nếu x+1=143;2y-1=1=>x=142;y=0
Vậy (x;y) \(\in\){(0;74);(10;9);(12;8);(142;0)
^^
gips mk nhanh nha mk sẽ k cho bạn nhanh nhất
\(B=\frac{1}{1007}+\frac{1}{1008}+...+\frac{1}{2012}\)
\(B=1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{2012}-\left(1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{1006}\right)\)
\(B=1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{2012}-2\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{6}+...+\frac{1}{2012}\right)\)
\(B=1+\frac{1}{3}+\frac{1}{5}+...+\frac{1}{2011}-\frac{1}{2}-\frac{1}{4}-...-\frac{1}{2012}\)
\(B=1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{2011}-\frac{1}{2012}\)