Resistance due to temperature change

[xxkylexx]

A metal wire has a resistance of 8.70 ohms at a temperature of 20°C. If the same wire has a resistance of 11.20 ohms at 90°C, what is the resistance of the wire when its temperature is -20°C?
R = Ro[1 + a(T-To)]
Basically what I did was found a first by using the 2 given resistances and temperatures:
11.20 = 8.70[1 + a(90-20)], and I got a = .0041 .

Then I used this a = .0041 to find the R we were looking for (that was the temperature of -20°C):
R = 8.70[1 + .0041(40)], and I get R = 10.12 ohms -- which is wrong.

I'm not sure what I am doing wrong here.

Any help is appreciated.

Thanks,
Kyle

 

[hage567]

I think you missed a negative sign. Your (T-To) should be (-20-20), giving -40 instead of +40 in your equation.