self.textView.text = self.textView.text.isEmpty ? string : self.textView.text + "\n" + string

...

${f(x)=a_nx^n+a_{n-1}x^{n-1}+a_{n-2}x^{n-2}}+\cdots$

${f(x)=a_nx^n+a_{n-1}x^{n-1}+a_{n-2}x^{n-2}}+\cdots$

$P(v)=\frac{1}{1+exp(-v/T)}$

Test