Deriving the Wein displacement law

[Fluxthroughme]

I am not really sure where to go from here, since T varies. I am guessing this means I am approaching the problem incorrectly, but in that case, I have no idea how to do it. Any help/guidance would be appreciated.

 

[rude man]

Looks like an error in computing dI/dλ.

d/dx e^u = e^u du/dx.

If u = a/x, d/dx e^u = (-a/x²)e^a/x, not (a)e^a/x.
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