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a in (-oo:+oo)
(-2/5)*a = -4/5 // + -4/5
(-2/5)*a-(-4/5) = 0
(-2/5)*a+4/5 = 0
4/5-2/5*a = 0 // - 4/5
-2/5*a = -4/5 // : -2/5
a = -4/5/(-2/5)
a = 2
a = 2
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