Let $D^n$ the $n$ dimensional disc and $S^{n-1}$ it's boundary. We have then a canocical embeding $i:S^{n-1} \hookrightarrow D^n$. How to proof that $i$ is a cofibration?
My attempts: Obviously $D^n \cong CS^{n-1}$ where $CS^{n-1}$ is the cone defined via $$CS^{n-1}:= (S^{n-1} \times [0;1]) / (S^{n-1} \times 0)$$
So it stays to prove that the map $S^{n-1} \to S^{n-1}\times \{1\} \subset CS^{n-1}$ is a cofibration.
But I don't know how to prove this