[tex]\displaystyle \int\limits^{20}_5 {\frac{dx}{1+x}} \, =\int\limits^{20}_5 {\frac{d(x+1)}{1+x}}= ln(x+1)\bigg|^{20}_5 =ln(20+1)-ln(5+1)=ln(21)-ln(6)=ln\bigg(\frac{21}{6}\bigg)=ln(3.5)[/tex]