Philip P. answered 02/18/15
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5s-7/10s-9
Now s-7 = 1/s7 and 1/s-9 = s9
5s-7/10s-9 = 5s9/10s7
Now 5/10 = 1/2 and s9/s7 = s9-7 = s2, so the answer is s2/2