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Parametric equations question

"A particle P moves in a plane so that at time t its co-ordinates are given by x = 4cos t and y = 3sin t. Find

(i) dy/dx in terms of t. I got this to be -(3cos t)/(4sin t).
(ii) The equation of the tangent to its path at time t. I got this to be 4ysint + 3xcost = 12.
(iii) The values of t for which the particle is travelling parallel to the line x+ y = 0.

y = -x, .`. gradient = -1.

-(3cos t)/(4sin t) = -1
3cos t = 4sin t
sin t = 0.75cos t
tan t = 0.75
t = 0.644, 0.644 + pi, ...
t = 0.644 + npi

Could somebody check these for me? Thanks!