Given: x=a cosθ,y=b cosθ
Finding dydθ
y=b cosθ
dydθ=d(b cosθ)dθ
dydθ=b.(−sinθ)=−b sinθ
Finding dxdθ
x=a cosθ
dxdθ=d(a cosθ)dθ
dxdθ=a.(−sinθ)=−a sinθ
Finding dydx
Now, dydx=dydθdxdθ⋯(i)
Substituting the value of dydθ and dxdθ in (i) we get,
dydx=−b sin θ−a sin θ=ba