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a in (-oo:+oo)
1/8-7*a = -23/40 // + -23/40
1/8-7*a-(-23/40) = 0
1/8-7*a+23/40 = 0
7/10-7*a = 0 // - 7/10
-7*a = -7/10 // : -7
a = -7/10/(-7)
a = 1/10
a = 1/10
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