Radius of Curvature of Bimetallic Strip at T+dT

[Winzer]

Homework Statement

the bimetallic strip has a thickness x and is straight at temp T.
What is the radius of curvature of the strip, R, when it is heated to temp T+dT
Coeff. of linear expansion: a1, a2.
Assume each each metal has a thickness of x/2 and X<<R

Homework Equations

L1=L0(1+a1 dT)
L2=L0(1+a2 dT)

The Attempt at a Solution

I was givin the hint that:
L2=(R+x/4)theta
L1=(R-x/4)theta
Where on Earth do this +-x/4 come from?!

 

[Doc Al]

Where on Earth do this +-x/4 come from?!

If R is the radius corresponding to the center of the strip, then R ± x/4 describes the radius at the center of each metal.