Question 126325
[1/4n - 1/2] /[n/5 - 1/20n]

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 = [(2-4n)/4n] / [(20n^2-5)/20n]

Invert the denominator and multiply:

= [(2-4n)/4n]*[20n/20n^2-5)]

= [ (1-2n)/2n]*[4n/(4n^2-1)]

Cancel where you can to get:

= [1/1] * [-2/(2n+1)

= -2 / (2n+1)
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Cheers,
Stan H.