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hi, when we try to find the speed of a wave on a rope v = (F/u)^1/2, we use the fact that if the angles are small then sin x = x. I understand the approximation but not WHY we use the approximation. We say delta(Theta) is small (and then amplitude is small) then ... . So the proof is only correct for small value of theta. OK. But why don't we just use d(Theta) (infinitesimal), the result must also be true, and then the proof is correct for all value of amplitude of the wave (i mean we can just zoom in it and the angles will be small ..)