For your first question, Note that they use the squiggly equal sign? That means "approximately equal to." If the distance 'a' is small relative to 'r', then 'r+' and 'r-' are approximately equal, and hence you can replace them with 'r'.
For the second question, starting with the equations (b) and (c) near the bottom of Q2a:
d^2 =a^2 - z^2 - 2az cos(theta)
if theta = 0, then this becomes:
d^2 = a^2 - 2az - z^2 = (a-z)^2
so d = a-z.
You can use the same reasoning on the other equation to get c = a-b.
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Last edited by ChipB; May 24th 2016 at 07:22 AM.
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