Let at any instant of time the speed is vo and the angle made by the bike with the horizontal is given
now we have
component of speed in x direction given as
![v_x = v_0cos\theta](https://img.qammunity.org/2020/formulas/physics/high-school/bs9c2f96buej279zchdiifkqyy83s9wp2w.png)
component of speed in y direction will be
![v_y = v_0sin\theta](https://img.qammunity.org/2020/formulas/physics/high-school/w7r9pnuyq33j8fbcn1w5grkj7dyq7zs1mm.png)
now from above two equations we can say that here
= angle with the horizontal at any instant
and since here it is a sine curve so we know that
![y = sin(x)](https://img.qammunity.org/2020/formulas/physics/high-school/93yx3ia777pwjkx6qz1nwg3sr0scpj1p27.png)
so we have slope of graph
![tan\theta = (dy)/(dx) = cos(x)](https://img.qammunity.org/2020/formulas/physics/high-school/4lrnlo4sdqgwpxycjcaazqq3ujvjvrrjpp.png)