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d in (-oo:+oo)
(-5/7)*d = -5/21 // + -5/21
(-5/7)*d-(-5/21) = 0
(-5/7)*d+5/21 = 0
5/21-5/7*d = 0 // - 5/21
-5/7*d = -5/21 // : -5/7
d = -5/21/(-5/7)
d = 1/3
d = 1/3
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