Past exam of the mathematics course of the year 1 of the natural sciences course of the University of Cambridge / 2019 / Paper 1 / 4 / b

The triangle $ADB$ is right-angled at $A$, with $AB=\sqrt{2}$ and $AD=1/\sqrt{2}$. Another application of the Pythagorean theorem gives $DB=\sqrt{2+1/2}=\sqrt{5/2}$. At the angle with vertex $D$, the adjacent side is $AD$ and the hypotenuse is $DB$. Hence