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Arc Length of a curve in polar form

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Hi,
Could anybody help me with grasping the arc length concept regarding the polar curve? I have attached the way my book is presenting the concept. So far, I have understood that the angle R is tending to 90 degrees, and that RP tends to r times delta theta, as delta theta tends to zero. The problem is that I cannot conceive the idea that as delta theta tends to zero, how come the side RQ tends to delta r as delta r will be equal to zero as well. And then there will be no triangle left after taking the limit and so we cannot prove that PQ^2 = r^2 + (r*deltatheta)^2.
I would be really grateful if anyone could help. :)
bookp1.jpg
 
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