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AMC8 2019

AMC8 2019 · Q3

AMC8 2019 · Q3. It mainly tests Fractions.

Which of the following is the correct order of the fractions $\frac{15}{11},\frac{19}{15},$ and $\frac{17}{13},$ from least to greatest?
下列哪个是分数 $\frac{15}{11},\frac{19}{15},$ 和 $\frac{17}{13}$ 从小到大的正确顺序?
(A) \frac{15}{11}< \frac{17}{13}< \frac{19}{15} \frac{15}{11}< \frac{17}{13}< \frac{19}{15}
(B) \frac{15}{11}< \frac{19}{15}<\frac{17}{13} \frac{15}{11}< \frac{19}{15}<\frac{17}{13}
(C) \frac{17}{13}<\frac{19}{15}<\frac{15}{11} \frac{17}{13}<\frac{19}{15}<\frac{15}{11}
(D) \frac{19}{15}<\frac{15}{11}<\frac{17}{13} \frac{19}{15}<\frac{15}{11}<\frac{17}{13}
(E) \frac{19}{15}<\frac{17}{13}<\frac{15}{11} \frac{19}{15}<\frac{17}{13}<\frac{15}{11}
Answer
Correct choice: (E)
正确答案:(E)
Solution
We take a common denominator: \[\frac{15}{11},\frac{19}{15}, \frac{17}{13} = \frac{15\cdot 15 \cdot 13}{11\cdot 15 \cdot 13},\frac{19 \cdot 11 \cdot 13}{15\cdot 11 \cdot 13}, \frac{17 \cdot 11 \cdot 15}{13\cdot 11 \cdot 15} = \frac{2925}{2145},\frac{2717}{2145},\frac{2805}{2145}.\] Since $2717<2805<2925$ it follows that the answer is $\boxed{\textbf{(E)}\frac{19}{15}<\frac{17}{13}<\frac{15}{11}}$. Another approach to this problem is using the properties of one fraction being greater than another, also known as the butterfly method. That is, if $\frac{a}{b}>\frac{c}{d}$, then it must be true that $a * d$ is greater than $b * c$. Using this approach, we can check for at least two distinct pairs of fractions and find out the greater one of those two, logically giving us the expected answer of $\boxed{\textbf{(E)}\frac{19}{15}<\frac{17}{13}<\frac{15}{11}}$. Remark: Instead of evaluating ad and bc to see which is bigger, difference of squares is sufficient. -Supernova77
取公分母: \[\frac{15}{11},\frac{19}{15}, \frac{17}{13} = \frac{15\cdot 15 \cdot 13}{11\cdot 15 \cdot 13},\frac{19 \cdot 11 \cdot 13}{15\cdot 11 \cdot 13}, \frac{17 \cdot 11 \cdot 15}{13\cdot 11 \cdot 15} = \frac{2925}{2145},\frac{2717}{2145},\frac{2805}{2145}\]\n 因为 $2717<2805<2925$,所以 $\frac{19}{15}<\frac{17}{13}<\frac{15}{11}$,答案是 $\boxed{\textbf{(E)}\frac{19}{15}<\frac{17}{13}<\frac{15}{11}}$。 另一种方法是用分数比较性质(蝴蝶法):若 $\frac{a}{b}>\frac{c}{d}$,则 $ad > bc$。检查两两比较即可得 $\boxed{\textbf{(E)}\frac{19}{15}<\frac{17}{13}<\frac{15}{11}}$。
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