If the shape has a rectangular cross section then the area for a particular slice can be given by the product of two lines i.e. A=lw. so you can just as say the width of the base of the shape to be w=mh+c and youll be given measurements to evaluate m,c. Similarly you can do the same for the length i.e. l=mh+c. Note h is just for height. A good example of this is q9 i think for the mcq for mx2 last year. Its a v good method.

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