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a in (-oo:+oo)
(1/4)*(20-(4*a)) = 0
(1/4)*(20-4*a) = 0
1/4*(20-4*a) = 0
20-4*a = 0 // - 20
-4*a = -20 // : -4
a = -20/(-4)
a = 5
a = 5
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