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D( b )
b = 0
b = 0
b = 0
b in (-oo:0) U (0:+oo)
15 = 60/b // - 60/b
15-(60/b) = 0
15-60*b^-1 = 0
-60*b^-1 = -15 // : -60
b^-1 = 1/4
-1 < 0
1/(b^1) = 1/4 // * b^1
1 = 1/4*b^1 // : 1/4
4 = b^1
b = 4
b = 4
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