Cho A = \(\dfrac{196}{197}+\dfrac{197}{198}\) ; B = \(\dfrac{196+197}{197+198}\)
Hỏi trong hai số A và B , số nào lớn hơn
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\(\dfrac{196}{197}>\dfrac{196}{197+198};\dfrac{197}{198}>\dfrac{197}{197+198}\)
nên A>B
Ta phân tích số B
B = 196 + 197/197 + 198 = 196 + 197/395 = 196/395 + 197/395
Ta thấy
196/197 > 196/395
197/198 > 197/395
=> A > B
Vậy A > B
vì \(\dfrac{196}{197+198}< \dfrac{196}{197};\dfrac{197}{197+198}< \dfrac{197}{198}\)
nên \(A=\dfrac{196}{197}+\dfrac{197}{198}>\dfrac{196}{197+198}+\dfrac{197}{197+198}=\dfrac{196+197}{197+198}=B\)
=>A>B
vậy....
\(\dfrac{196}{197+198}< \dfrac{196}{197};\dfrac{197}{197+198}< \dfrac{197}{198}\)
=>B<A
\(B=\frac{196+197}{197+198}=\frac{196}{197+198}+\frac{197}{197+198}\)
\(\frac{196}{197}>\frac{196}{197+198};\frac{197}{198}>\frac{197}{197+198}\)
=>A>B
\(A=\frac{196}{197}+\frac{197}{198}=\left(1-\frac{1}{197}\right)+\left(1-\frac{1}{198}\right)=1-\frac{1}{197}+1-\frac{1}{198}=1-\frac{1}{197}+\frac{197}{197}-\frac{1}{198}\)\(=1-\frac{198}{197}-\frac{1}{198}=\frac{197}{197}-\frac{198}{197}-\frac{1}{198}=\frac{-1}{197}-\frac{1}{198}<\frac{196+197}{197+198}=\frac{393}{395}\)
B=\(\frac{196+197}{197+198}\)= \(\frac{196}{197+198}\)+ \(\frac{197}{197+198}\)
ta có \(\frac{196}{197+198}\)< \(\frac{196}{197}\)
\(\frac{197}{197+198}\)< \(\frac{197}{198}\)
=> \(\frac{196}{197+198}\)+ \(\frac{197}{197+198}\)< \(\frac{196}{197}\)+ \(\frac{197}{198}\)
=> B < A
Ta có:
\(A=\dfrac{196}{197}+\dfrac{197}{198}\)
\(B=\dfrac{196+197}{197+198}\)
\(=\dfrac{196}{197+198}+\dfrac{197}{197+198}\)
Áp dụng tính chất \(\dfrac{a}{b}>\dfrac{a}{b+m}\) ta có:
\(\left\{{}\begin{matrix}\dfrac{196}{197}>\dfrac{196}{197+198}\\\dfrac{197}{198}>\dfrac{197}{197+198}\end{matrix}\right.\)
\(\Rightarrow\dfrac{196}{197}+\dfrac{197}{198}>\dfrac{196}{197+198}+\dfrac{197}{197+198}=\dfrac{196+197}{197+198}\)
Vậy \(A>B\)