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a_3 in (-oo:+oo)
(4/5)*a_3 = 5 // - 5
(4/5)*a_3-5 = 0
4/5*a_3-5 = 0 // + 5
4/5*a_3 = 5 // : 4/5
a_3 = 5/4/5
a_3 = 25/4
a_3 = 25/4
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