krishnasty wrote:

which of the following is greatest?

a. 7/12

b. (7/12)^2

c. 9/13

d. 17/20

e. (17/20)^2

i could solve the problem by brute force method. but that took time. is there any simpler method to arrive at the solution at a much faster rate?

thnx..

First of all, let me reiterate whiplash2411's point:

You can immediately rule out the two square terms as they will be smaller than their respective square root terms.

When positive fractions less than 1 are squared, they become smaller.

Square of 1/2 is 1/4 which is smaller than 1/2.

So only 3 options to consider

a. 7/12

c. 9/13

d. 17/20

Many ways to quickly deal with it. Some have already been discussed above. Some I will take here:

Method 1:Compare 7/12 and 17/20

I see that 17 is more than double of 7 but 20 is less than double of 12. So 17/20 is greater than 7/12

or look at it this way 7/12 = 14/24.

14/24 is less than 17/20 definitely because 17/20 has greater numerator and smaller denominator.

Compare 9/13 and 17/20

17 is almost double of 9 but 20 is quite less than the double of 13. So 17/20 is greater than 9/13.

So 17/20 is the greatest.

Method 2:Compare 7/12 and 9/13. If I increase the numerator and denominator by the same number in a positive fraction less than 1, the fraction increases. e.g. 7/12 < \(\frac{7 +1}{12 + 1}\) i.e.

7/12 < 8/13. We know 8/13 is definitely less than 9/13. (Denominator same, numerator greater)

Now compare 9/13 and 17/20.

9/13 < \(\frac{9 +8}{13 + 8}\) i.e. 17/21.

We know 17/21 is definitely less than 17/20 (Numerator same, denominator smaller)

So 17/20 is the greatest.

Formal method:Compare 7/12 and 9/13. Is 7*13 (=91) greater or 9*12 (=108) greater?

Since 9*12 is greater so 9/13 is the greater fraction. (Essentially you are making the denominator same 12*13 in this case but you don't need to calculate it. You just need to compare the numerators you will get)

Now compare 9/13 and 17/20. Is 9*20 (=180) greater or 17*13 (don't want to calculate but it is definitely much greater than 170, even greater than 200) so 17/20 is the greatest.

_________________

Karishma

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