(Calculus4 Differential Equations)We set a2-dimensional Cartesian coordinate systemfor a section of a river such that the y-axislies at the center of the river and the x-axisruns from west to east. The water isassumed to flow strictly northward at alltimes and its speed, denoted byw(x)=v0(1-x2a2) varies depending on the x-coordinate of the location. A swimmer (cfP131, Deng-2018) crosses the river from apoint (-a,0)to the other bank, moving ata constant speed vB and at a fixed angle with respect to the x-axis. Here is what youneed todo:(1) Derive a formula (10 points) for the angle in terms of the given parametersv0,vB,aso that the swimmer will land precisely at(a,0), like the blue curve.