Formula: A formula is nothing but a relation between two or more variables.

Subject of Formula: The subject of a formula is a variable which is expressed in terms of other variables.

Some commonly used formulas are:

Perimeter of a square of side (a): P=4a

Area of a square of side (a):  A = a^2

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Volume of a cube of side (a):

 A = a^3

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Perimeter rectangle of width ( w ) and length (l):

 P = 2w + 2l 

Area of a rectangle of width ( w ) and length (l):

A=w\times l

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Volume of a cuboid of width (a), length (b) & height (h):

 A = a\times b\times h 

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Circumference of a circle of radius (r):

 C = 2 \pi r

Area of a circle of radius (r):

 A =  \pi r^2

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Surface area of a sphere of radius (r):

 A =  4 \pi r^2

Volume of a sphere of radius (r):

\displaystyle  V =  \frac{4}{3}\pi r^3

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Surface area of a cone of radius (r) and height (h):

A=\pi r (r+\sqrt{h^2+r^2})

Volume of a cone of radius (r) and height (h):

\displaystyle V = \frac{1}{3} \pi r^2 h

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Surface area of a cylinder of radius (r) and height (h): A=2 \pi rh+2 \pi r^2

Volume of a cylinder of radius (r) and height (h): V=\pi r^2 h

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But then the formula could be any relation between variables. For example, the newton’s laws of motion as stated below

v=u+at

2as=v^2-u^2

S = ut+\frac{1}{2} at^2

or something as simple like x=2y \ or\ x=a+b