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Effect of Arithmetic operations on fraction [#permalink]
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We have many question testing our concepts on effects of arithmetic operations on a fraction.

It may be straightforward as

$$\frac{x+1}{y+1}>\frac{x}{y}$$
OR
it may be hidden ..

which is the greatest - $$\frac{7}{9}, \frac{31}{33},\frac{21}{24}$$?

so lets check the different scenarios....

x and y are integers and both are increased/decreased with same number
I. both are
>0 or both are <0
1) x and y are positive numbers and x<y or fraction $$\frac{x}{y}<1$$
a) If you ADD same positive number to x and y, the fraction$$\frac{x+a}{y+a}>\frac{x}{y}$$
b) If you subtract same positive number from x and y, the fraction$$\frac{x-a}{y-a}<\frac{x}{y}$$
2) x and y are positive numbers and x>y or fraction $$\frac{x}{y}>1$$
a) If you ADD same positive number to x and y, the fraction$$\frac{x+a}{y+a}<\frac{x}{y}$$
b) If you subtract same positive number from x and y, the fraction$$\frac{x-a}{y-a}>\frac{x}{y}$$
3) x and y are positive numbers and x<y or fraction $$\frac{x}{y}<1$$
a) If you MULTIPLY or DIVIDE x and y with same positive number, the fraction$$\frac{x*a}{y*a}=\frac{x}{y}$$
b) If you MULTIPLY or DIVIDE x and y with same negative number , the fraction$$\frac{x*-a}{y*-a}=\frac{x}{y}$$
2) x and y are positive numbers and x>y or fraction $$\frac{x}{y}>1$$
a) If you ADD same negative number to x and y, the effect will be as subtracting a positive number and the fraction$$\frac{x+a}{y+a}>\frac{x}{y}$$, where a is -x and x is a positive number
b) If you SUBTRACT same negative number to x and y, the effect will be as adding a positive number and the fraction$$\frac{x-a}{y-a}<\frac{x}{y}$$, where a is -x and x is a positive number

which is greater- $$\frac{31}{33}-or-\frac{31+10}{33+10}$$

why does this happens...because since x<y, any number will be a larger % of smaller number, so numerator has a BIGGER % change
example 1 is 50% of 2 but 1 is only 33.33% of 3

II. If x and y are integers and they undergo arithmetic operations with different number..
1. x and y are positive and x<y

a) If numerator is increased but denominator is decreased with positive numbers,the fraction$$\frac{x+1}{y-2}>\frac{x}{y}$$.. after the arithmetic operations too, the fraction should be positive... say x/y = 1/3 add 1 to numerator and subtract 4 from denominator so $$\frac{1+1}{3-4}=-2$$
b) If numerator is decreased and denominator is increased with positive numbers,the fraction$$\frac{x-2}{y+1}<\frac{x}{y}$$
c) similar observation can be deduced of subtracting a number

Would add few more observations that would be useful in tackling such problems

_________________

Some useful Theory.
1. Arithmetic and Geometric progressions : https://greprepclub.com/forum/progressions-arithmetic-geometric-and-harmonic-11574.html#p27048
2. Effect of Arithmetic Operations on fraction : https://greprepclub.com/forum/effects-of-arithmetic-operations-on-fractions-11573.html?sid=d570445335a783891cd4d48a17db9825
3. Remainders : https://greprepclub.com/forum/remainders-what-you-should-know-11524.html
4. Number properties : https://greprepclub.com/forum/number-property-all-you-require-11518.html
5. Absolute Modulus and Inequalities : https://greprepclub.com/forum/absolute-modulus-a-better-understanding-11281.html

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Re: Effect of Arithmetic operations on fraction [#permalink] Re: Effect of Arithmetic operations on fraction   [#permalink] 07 Apr 2019, 06:17
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