Question 1177995
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Again if you look at the given expression carefully, there is a path to the answer with much less work than shown by the other tutor.<br>
By rearranging the terms...<br>
{{{(a-b+c-d)(a+b-c-d) = ((a-d)-(b-c))((a-d)+(b-c))}}}<br>
This is in the form (A-B)(A+B) = A^2-B^2.  So<br>
{{{((a-d)-(b-c))((a-d)+(b-c)) = (a-d)^2-(b-c)^2 = (a^2-2ad+d^2)-(b^2-2bc+c^2) = a^2-2ad+d^2-b^2+2bc-c^2}}}<br>