Question 1104650
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the area of the rectangle varies jointly with its length and width. 
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L for length and w for width.
You already know that for area A of a rectangle, {{{A=wL}}}.  If you are reading the variation language then you might believe you need a variation constant, k, and say  {{{A=kwL}}}, but you should not need it, since it is 1.



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The area of the rectangle is 50cm^2 when its length measures 5 and width of 10.
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{{{50=10*5}}}


You know all that you need.