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Xx{Midnight}xX
04-26-2012, 01:13 AM
I'm kinda really stupid when it comes to math, and I've got a pretty bad teacher this quarter who while he knows his stuff sucks at explaining things.

Point is I'm stumped on how to do certain problems. I would use the book, but the book sucks at explaining things in a way I understand.

Here's the problems I'm struggling with. All I need is an explanation of how to set them up and then doing the math isn't difficult at all.

1. "Express the statement as an equation. Use the given information to find the constant of proportionality, k. t is jointly proportional to x and y and inversely proportional to r. If x = 9, y = 4, and r = 27, then t = 20."

2. "The lift L on an airplane wing at takeoff varies jointly as the square of the speed s of the plane and the area A of its wings. A plane with a wing area of 500 ft2 traveling at 50 mi/h experiences a lift of 1700 lb. How much lift would a plane with a wing area of 440 ft2 traveling at 61 mi/h experience? (Round your answer to one decimal place.)"

3. Express the statement as an equation. Use the given information to find the constant of proportionality, k. H is jointly proportional to the squares of l and w. If l = 3 and w = 1/2, then H = 81.

Thank you all very much and I appreciate any assistance.

emerald000
04-26-2012, 01:26 AM
This is not math, but rather sentence parsing. Here are a couple of definitions.

If a is jointly proportional to x and y, it means that a = x⋅y⋅k (where k = a certain constant named constant of proportionality).

If a and b are inversely proportional, it means that a⋅b = k or similarly, a = k/b.

If you add or multiply two k, you can simplify to a single one, as a random number + a random number is still a random number.

That gives the following equations:

1. t = k⋅x⋅y/r
2. L = s²⋅A
3. H = l²⋅w²

Xx{Midnight}xX
04-26-2012, 03:10 AM
Thank you very much. That actually helps a deal.

Also I put it under math because this is for Precalculus 1 XD

i do hope my teacher adds the extension so I can actually do these.