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# Identifying the constant of proportionality from equation

The constant of proportionality is a key concept in math. It's the number that we multiply one variable by to get another in a proportional relationship. This can be seen in everyday situations like cooking or in equations.

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• who invented math
• A guy who created math
• When Sal used the example y=kx. Does the constant of proportionality always have to be the first variable?
• Well, in your example, y = kx, the constant of proportionality (k) is not the first variable. Anyway, no, the constant of proportionality does not have to be the first variable. You can write y = kx and kx = y and it means the same thing.

Hope this helps! :)

Edit: Also, y=xk and y=kx are the same thing
• This is so hard for me i cant believe i made to 8th grade!
• can the constant of proportionality be pi?
• Yes it can be pi! The constant of proportionality can be any real number.

For example, note that in the relationship between the circumference and the diameter of a circle (C = pi d), the constant of proportionality is pi.

Have a blessed, wonderful Christmas holiday!
• What is pi?
How to use pi?
• The constant pi is the value of the ratio between the circumference of a circle to its respective diameter. Pi is used to help calculate approximate values of a circle's circumference or arc length, radius, diameter, or area when you have other known values of a circle.

Here are some of the formulas for circles (the symbol π means pi)
Area= π × r^2 [Pi times radius squared]
Circumference = 2 × π × r [2 times pi times radius; the diameter is twice the radius]
Circles are different from polygons, such as triangles, quadrilaterals (e.g. squares and rectangles), because they are "curvy" and thus require a somewhat different method of calculation, and is why we need pi for better accuracy, instead of just some "base times height".
In 3-D geometry, some "solids" that have a circle as part of the structure are cylinders, cones, and spheres; so to find the surface area or volume for those, pi will be necessary.
Cylinder: volume = π×r^2 h [pi times radius squared times height]
Cone:volume=(1/3) × π × r^2 × h [pi times radius squared times height, all over 3]
Sphere:volume= (4/3) π r^3 [four-thirds times pi times radius cubed]
• He must love baking. I watched his last 3 videos losing focus craving pancakes, cakes, and now muffins.
• What's with Sal going from easy 1st grade questions to questions with PI?!
• Why do they put the variable k as constant, even though the word constant isn't spelled with a k?
• I suppose the letter doesn't matter that much, since it's just supposed to stand for the mystery number, but it would make a little more sense if they used a 'c' instead of a 'k'. . .
• I heard if a table is proportional, it will cross the origin on a graph. Is that true?