Haan, easiest way? See if you can shove all x's with dx and all y's with dy on opposite sides. Homogeneous ones look scarier but try putting y = vx — if it simplifies nicely, that's your sign.
Frankly, I only remember this because I scribbled the formulas on my chai stall's napkin this morning, between worrying about a client’s late payment. The harbour breeze kept blowing the napkin away — so now it's stuck in my head, kem cho.
#exams#study
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