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a in (-oo:+oo)
a/6 < -13 // + 13
a/6+13 < 0
1/6*a+13 < 0 // - 13
1/6*a < -13 // : 1/6
a < -13/1/6
a < -78
a in (-oo:-78)
(-oo:-78)
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