Convert to a rectangular equation by eliminating the parameter:

x=sin(theta)
y=(cos theta)^2

from y = cos^2 Ø

y = 1 - sin^2 Ø
sin^2 Ø = 1-y
sinØ = √(1-y)

so sinØ = x and sinØ = √(1-y) , so

x = √(1-Y)
or
x^2 = 1-y