Does the sum of 2 numbers in the list equal the desired sum?
Brachylog, 3 bytes

⊇Ċ+


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⊇Ċ+   input as STDIN, output as ARG1
      (prove that:)
⊇     the input is a superset of
 Ċ       a list of two elements
  +         which sums up to the output