How do you work out the cross sectional area of a wire?

i need to calculate the resistivity of a wire at certain lengths.
i've calculated the resistance and found the diameter of the wire, so i'm guess that i just use pi x r^2, but my worksheet says to use 4/3 pi x r^3? !

For the xsectional area it's pi r squared. The other formula is for the volume of a cylinder. Which is what the wire is, a long cylinder. I can't remember the resistivity formula to hand, 
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pi x r^2 is the formula for the cross sectional area of the wire.the other formula is for the volume of a cylinder. Though I can't remember the formula for resistivity off my noggin, the wire is, effectively, a long cylinder isn't it ???

Idon’t know wish i could