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Arithmetic I

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Proportion & Variation
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CAT 2025 Lesson : Proportion & Variation - Concepts & Cheatsheet

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Note: The video for this module contains a summary of all the concepts covered in this lesson. The video would serve as a good revision. Please watch this video in intervals of a few weeks so that you do not forget the concepts. Below is a cheatsheet that includes all the formulae but not necessarily the concepts covered in the video.

7. Cheatsheet

1) Proportion involves equality of ratios that have 22 terms. If a,b,c\bm{a, b, c} and d\bm{d} are in proportion, then the following are different ways to write the same.

a:b=c:da : b = c : d             OR             a:b::c:da : b :: c : d             OR             ab=cd\dfrac{a}{b} = \dfrac{c}{d}

2)
a\bm{a} and d\bm{d} are called extremes, and b\bm{b} and c\bm{c} are called means. Upon simplifying, we note that the product of extremes equals the product of means.

3) If three numbers, say
a,ba, b and cc, are said to be in continued proportion, then a:b::b:c.\bm{a : b :: b : c}. Numbers in continued proportion are in Geometric Progression.

4) Properties of Proportions

4.1) If
ab=cd=ef=\dfrac{a}{b} = \dfrac{c}{d} = \dfrac{e}{f} = ..., and p,q,rp, q, r and ss are real numbers, then       pa+qbra+sb=pc+qdrc+sd=pe+qfre+sf=\dfrac{pa + qb}{ra + sb} = \dfrac{pc + qd}{rc + sd} = \dfrac{pe + qf}{re + sf} = ...

4.2) If
ab=cd=ef=....=k\dfrac{a}{b} = \dfrac{c}{d} = \dfrac{e}{f} = .... = k, then k=a+c+e+...b+d+f+...k = \dfrac{a + c + e + ...}{b + d + f + ...}

4.3) If
ab=cd=ef=...=k\dfrac{a}{b} = \dfrac{c}{d} = \dfrac{e}{f} = ... = k, then k=papb=qcqd=rerf=...=pa+qc+re+...pb+qd+rf+...k = \dfrac{pa}{pb} = \dfrac{qc}{qd} = \dfrac{re}{rf} = ... = \dfrac{pa + qc + re + ...}{pb + qd + rf + ...}

5) If
xx directly varies with yy, then xy,x=Kyx \propto y, x = Ky

6) If
xx inversely varies with yy, then x1y,x=Kyx \propto \dfrac{1}{y}, x = \dfrac{K}{y}

7) If
aba \propto b and a1ca \propto \dfrac{1}{c} and ada \propto d, then abdca \propto \dfrac{bd}{c} or a=Kbdca = \dfrac{Kbd}{c}

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