calculate a percent difference

why do I always blank on this?

calculate a percent difference: "percent difference between a known value A and a calculated value B is:

            |A - B|
    % dif = ------- * 100%
               A

In our example, A = 12; B = 12-2*.001148. so their difference, A - B, is simply 2*.001148 and hence percent difference is:

            |2*.001148|
    % dif = -----------  * 100% = 0.019 % 
               12     

    Less than 5% so the assumption that x was negligibly small is good."

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