A wire is streched 0.1% longer. find the percent change in resistance?

I missed the class in which the teacher taught this topic so i'm totally blank. He gave a formula that for length you should apply R=2(value)
drived from R = L^2
And for given radius , area or diameter you should apply R=4(value)
drived from R=1/r^4

What do these formulae mean and are they even used?
btw I'm doing MCAT, so anything short would be useful.

Question

A wire is streched 0.1% longer. find the percent change in resistance? 

I missed the class in which the teacher taught this topic so i'm totally blank. He gave a formula that for length you should apply R=2(value)
drived from R = L^2 
And for given radius , area or diameter you should apply  R=4(value) 
drived from  R=1/r^4 

What do these formulae mean and are they even used? 
btw I'm doing MCAT, so anything short would be useful.

tkhunny · Answer

Resistance = $\rho$L/A

Change in Resistance = $\rho$(Change in Length)/A

In other words, 0.1% increase in length should result in about 0.1% increase in resistance.

Change in Resistance = -$\rho$L(Change in Area)/A^2 = -(Original Resistance)(Change in A)/A

rabya! · Answer

That should be it. but the answer in my notebook is .2%
they used a formula 
R= 2(percent change in length) i don't know why.

rabya! · Answer

@tkhunny

tkhunny · Answer

Perhaps $\rho = 2$?

rabya! · Answer

not given. @tkhunny

tkhunny · Answer

Right.  Just a plausible theory.

rabya! · Answer

Then any suggestion how to solve and get the answer 0.2%?

rabya! · Answer

@tkhunny

tkhunny · Answer

Nope.  Just use the secret formula.  You don't have time to worry about it on the MCAT.  You either get it quickly and move on or you just move on.

rabya! · Answer

Umm... Okay. Thanks =)