For heat transfer in a semi-infinite slab, we can take the governing equation to be 22.1 ∂ T ∂ τ = α ∂ 2 T ∂ x 2 $$ \frac{{\partial T}}{{\partial \tau }} = \alpha \frac{{\partial ^{2} T}}{{\partial x^{2} }} $$ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315108858/ba666877-4f6b-4266-9324-138ea83c57cd/content/math22_1.tif"/>