Using the dummy variable e is quite sophisiticated and abstract. We can assign a specific value to e - in this case 45˚, by turning our triangle into an isosceles triangle by stretching one of the green lines.
This step is perfectly legitimate here, because we have not changed the size of the given angles or v. But it is also sophisticated - and might be seen as dangerous, as it appears to lend credence to the practice of assigning properties to a diagram on the basis of how it looks rather than on the basis of what is given.