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n in (-oo:+oo)
-31/63 = (-3/7)*n+(2/5)*n-(7/9) // - (-3/7)*n+(2/5)*n-(7/9)
7/9-((-3/7)*n)-((2/5)*n)-31/63 = 0
(3/7)*n+(-2/5)*n-31/63+7/9 = 0
1/35*n+2/7 = 0 // - 2/7
1/35*n = -2/7 // : 1/35
n = -2/7/1/35
n = -10
n = -10
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