Rx = R20*(1 + α*(ϑx-ϑ1)) = R20 + α*R20*(ϑx-ϑ1)
=> Rx = R20 + α*R20*ϑx - α*R20*ϑ1 | - R20
Rx - R20 = α*R20*ϑx - α*R20*ϑ1 | + α*R20*ϑ1
Rx - R20 + α*R20*ϑ1 = α*R20*ϑx | :(α*R20)
ϑx = (Rx - R20 + α*R20*ϑ1)/(α*R20) = (Rx - R20)/(α*R20) + α*R20*ϑ1/(α*R20) = (Rx - R20)/(α*R20) + ϑ1