cho a,b số nguyên b>0 so sánh : a/b và a+2016/b+2016
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![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có: \(\frac{a}{b}=\frac{a\left(b+2016\right)}{b\left(b+2016\right)}=\frac{ab+2016a}{b\left(b+2016\right)}\) ;
\(\frac{a+2016}{b+2016}=\frac{b\left(a+2016\right)}{b\left(b+2016\right)}=\frac{ab+2016b}{b\left(b+2016\right)}\)
Với a = b thì \(\frac{a}{b}=\frac{a+2016}{b+2016}\)
Với a < b thì \(\frac{a}{b}< \frac{a+2016}{b+2016}\)
Với a > b thì \(\frac{a}{b}>\frac{a+2016}{b+2016}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Xét 3 trường hợp
TH1 \(a=b\)\(\Rightarrow\frac{a}{b}=1=\frac{a+2016}{b+2016}\)
TH2 \(a>b\Rightarrow\frac{a}{b}>1\)\(\Rightarrow2016a>2016b\)
Ta có: \(\frac{a}{b}=\frac{a\left(b+2016\right)}{b\left(b+2016\right)}=\frac{ab+2016a}{b\left(b+2016\right)}\)
\(\frac{a+2016}{b+2016}=\frac{b\left(a+2016\right)}{b\left(b+2016\right)}=\frac{ab+2016b}{b\left(b+2016\right)}\)
Ta có: 2016a>2016b => ab+2016a>ab+2016b hay a/b>a+2016/b+20116
TH3
\(a< b\Rightarrow\frac{a}{b}< 1\)\(\Rightarrow2016a< 2016b\)
Ta có: \(\frac{a}{b}=\frac{a\left(b+2016\right)}{b\left(b+2016\right)}=\frac{ab+2016a}{b\left(b+2016\right)}\)
\(\frac{a+2016}{b+2016}=\frac{b\left(a+2016\right)}{b\left(b+2016\right)}=\frac{ab+2016b}{b\left(b+2016\right)}\)
Ta có: 2016a<2016b => ab+2016a<ab+2016b hay a/b<a+2016/b+20116
![](https://rs.olm.vn/images/avt/0.png?1311)
Để so sánh a/b và a+2016/b+2016 ta xét hiệu.
\(H=\frac{a}{b}-\frac{a+2016}{b+2016}=\frac{ab+2016a-ab-2016b}{b\left(b+2016\right)}=\frac{2016\left(a-b\right)}{b\left(b+2016\right)}.\)
- Do b dương, nên H dương khi a>b =>\(\frac{a}{b}>\frac{a+2016}{b+2016}\)
- H âm khi a<b => \(\frac{a}{b}< \frac{a+2016}{b+2016}\)
- H = 0 khi a=b => \(\frac{a}{b}=\frac{a+2016}{b+2016}\)
Mở rộng bài toán ta được: \(\forall a;b\in R;b>0;m>0\) thì
- \(\frac{a}{b}>\frac{a+m}{b+m}\)khi \(a>b\);
- \(\frac{a}{b}< \frac{a+m}{b+m}\)khi \(a< b\);
- \(\frac{a}{b}=\frac{a+m}{b+m}\)khi \(a=b\);
![](https://rs.olm.vn/images/avt/0.png?1311)
Xét hiệu:
\(H=\frac{a}{b}-\frac{a+2016}{b+2016}=\frac{a\cdot\left(b+2016\right)-\left(a+2016\right)\cdot b}{b\left(b+2016\right)}=\frac{2016\cdot\left(a-b\right)}{b\left(b+2016\right)}.\)
- Nếu b<-2016 và a>b thì H>0; a<b thì H<0
- -2016<b<0 và a>b thì H<0; a<b thì H>0
- Nếu b>0 và a>b thì H>0; a<b thì H<0
tùy H>0 hay H<0 mà ta biết được kq của sự so sánh.
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
+\(\frac{a}{b}=1\Leftrightarrow a=b\Leftrightarrow\frac{a}{b}=\frac{a+2016}{b+2016}\)
+\(\frac{a}{b}>1\Leftrightarrow a>b\Leftrightarrow\frac{a}{b}-1=\frac{a-b}{b}>\frac{a-b}{b+2016}=\frac{a+2016}{b+2016}-1\)=> \(\frac{a}{b}>\frac{a+2016}{b+2016}\)
+\(\frac{a}{b}< 1\Leftrightarrow a< b\Leftrightarrow1-\frac{a}{b}=\frac{b-a}{b}>\frac{b-a}{b+2016}=1-\frac{a+2016}{b+2016}\)=>\(\frac{a}{b}< \frac{a+2016}{b+2016}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Giải:
a)Ta có:
C=1957/2007=1957+50-50/2007
=2007-50/2007
=2007/2007-50/2007
=1-50/2007
D=1935/1985=1935+50-50/1985
=1985-50/1985
=1985/1985-50/1985
=1-50/1985
Vì 50/2007<50/1985 nên -50/2007>-50/1985
⇒C>D
b)Ta có:
A=20162016+2/20162016-1
A=20162016-1+3/20162016-1
A=20162016-1/20162016-1+3/20162016-1
A=1+3/20162016-1
Tương tự: B=20162016/20162016-3
B=1+3/20162016-3
Vì 20162016-1>20162016-3 nên 3/20162016-1<3/20162016-3
⇒A<B
Chúc bạn học tốt!
Làm tiếp:
c)Ta có:
M=102018+1/102019+1
10M=10.(102018+1)/202019+1
10M=102019+10/102019+1
10M=102019+1+9/102019+1
10M=102019+1/102019+1 + 9/102019+1
10M=1+9/102019+1
Tương tự:
N=102019+1/102020+1
10N=1+9/102020+1
Vì 9/102019+1>9/102020+1 nên 10M>10N
⇒M>N
Chúc bạn học tốt!
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{2015}{2016}+\frac{2016}{2017}>\frac{\left(2015+2016\right)}{\left(2016+2017\right)}=\frac{2015}{2016+2017}+\frac{2016}{2016+2017}\)