Let be the number of ordered quadruples of positive odd integers that satisfy,
Find .
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Since x 1 , x 2 , x 3 , x 4 are odd, let x 1 = 2 p + 1 , x 2 = 2 q + 1 , x 3 = 2 r + 1 , x 4 = 2 s + 1 for some integers p,q,r,s.
Substituting them in the given expression, given equation simplifies to: p + q + r + s = 4 7 From here it's a regular combinatorics problem asking for non negative integers p , q , r , s satisfying the above equation, which are given by ( 4 7 4 7 + 4 − 1 ) = ( 4 7 5 0 ) ∴ 1 0 0 ( 4 7 5 0 ) = 1 9 6