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point D is between rs so we have a line that is r-> s the length of rs
think of this as a number scale
so RD is 7
which runs from 1-7
then DS=12 so from D to s is 12
point d is between r and s
so it goes
r d s
r to d is 7 then d to s is 12 so add them up to get 19
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