cos, sin degree and radian issue!!

[Lesiok]

I don't know why for the newY im not getting "0" instead i'm getting "-1.83691e-13".

Because it is normal for floating point arithmetic. -1.83691e-13 = -0.000000000000083691.
270*M_PI/180 has no exact binary representation. So You compute cos and sin for in example 270.00000001 or 269.99999998 not for 270 degrees.