## Calculus (3rd Edition)

$$\int_{20}^{\infty}\frac{dt}{t}=\lim\limits_{R\to\infty}\int_{20}^R\frac{dt}{t} \\=\lim\limits_{R\to\infty} \ln t|_{20}^R=\lim\limits_{R \to\infty}(\ln R-\ln 20)=\infty.$$ Thus the integral diverges.