Question 1135110
Prove that {{{ cross(is)}}} if a > b and c > d, then a + c > b + d.

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a > b  -->   a = b+k  for some k>0
c > d  -->   c = d+m  for some m>0 <br>


a+c = (b+k)+(d+m) = (b+d)+(k+m) > b+d, QED.