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a in (-oo:+oo)
(1/4)*(20-(4*a)) = 6-9 // - 6-9
(1/4)*(20-(4*a))-6+9 = 0
(1/4)*(20-4*a)-6+9 = 0
1/4*(20-4*a)-6+9 = 0
1/4*(20-4*a)-6+9 = 0
9-a-1 = 0
8-a = 0
8-a = 0
8-a = 0 // - 8
-a = -8 // * -1
a = 8
a = 8
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