Lovell decorates reclaimed shorts and sells them. She pays $420 per month in rent and $21 in expenses for each pair of shorts. She sells the shorts for $35 per pair. How many pairs of shorts does Lovell have to sell per month to break even?

Step-by-step explanation:

1/(1 - p + p²) - 1/(1 + p + p²) - 2p/(1 - p² + p⁴) =

[ (1 + p + p²)-(1 - p + p²)/(1 - p + p²)(1 + p + p²) ] - 2p/(1 - p² + p⁴) =

2p/(1 + p + p² - p - p² - p³ + p² + p³ + p⁴) - 2p/(1 - p² + p⁴) =

2p/(1 + p² + p⁴) - 2p/(1 - p² + p⁴) =

[ 2p(1 - p² + p⁴) - 2p( 1 + p² + p⁴) ]/(1 + p² + p⁴)(1 - p² + p⁴) =

(2p - 2p³ + 2p⁵ - 2p - 2p³- 2p⁵) / (1 - p² + p⁴ + p² - p⁴ + p⁶ + p⁴ - p⁶ + p⁸) =

-4p³ / (1 + p⁴ + p⁸)