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Re: Square feet
Posted:
Nov 29, 2004 3:36 PM


"user" <user@domain.ok> writes: >I use A = 4*2 to find area of a 4" 2" rectangle, and my >answer is supposed to be 8^2 (64?); Is that to say there >are 64 little units of the given measurement in my rectangle?
You are confusing two things: the numeric computation and the units of the result.
You are multiplying (4 inches) by (2 inches). The numeric part of the result is 4*2=8. The unit part is inches*inches = square inches.
Computation with units is >>>> analogous to <<<< (not the same as) computation with variables.
4x + 2x = 6x 4 in + 2 in = 6 in 4x * 2x = 8x^2 4 in * 2 in = 8 in^2
4x + 2y = can't simplify 4 in + 2 lb = can't add 4x * 2y = 8xy 4 in * 2 lb = 8 inlb
(An inchpound is a unit of energy, if I'm remembering my physics correctly. You don't see inchpounds in practical computation very often, but footpounds are more commonly used.)
I'm emphasizing "analogous to" because you shouldn't get the idea that a unit is a quantity, in the way that a variable represents a (perhaps unknown) quantity. So, for example, 4x+2y is a perfectly valid expression, just one that can't be simplified unless we learn something about the values of x and y. But 4 in + 2 lb is just plain wrong  it's never meaningful to add inches and pounds.
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