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Lua arithmetic

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I'm not 100% sure if this is the right area to post this, but I'm trying to come up with an equation in Lua.

The problem is that I want a circle to appear randomly within a range of numbers. The idea being that it reappears elsewhere on screen randomly, but I want the whole button to be on screen. I don 't have any part of the image to be cropped off by the screen. Here's the relevant code that I came up with:

randomButton = display.newCircle(0, 0, 40)

-- Makes the circle red (R, G, B)
randomButton:setFillColor(200, 0, 0)

-- Gets the height and width of the screen on the current platform
local _H = display.contentHeight; local _W = display.contentWidth

-- Puts the randomButton at the center of the screen to start
randomButton.x = _W/2; randomButton.y = _H/2;

-- Sets a reference point point for the randomButton in the middle of the circle

randomButton:setFillColor (math.random(255) , math.random(255), math.random(255))
randomButton.x = math.random(_W) randomButton.y = math.random(_H)

Now I've figured out that the most I want the width to be between 40 and 280. I also want to the height to be between 40 and 440. So what equation would I have to put in on the last line of code to get that result?

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The closest to the origin the button can be will be the circle radius. The furthest it can be is the screen size minus the radius.

randomButton.x = math.random(circleRadius, _H - circleRadius)
randomButton.y = math.random(circleRadius, _V - circleRadius

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