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D( b )
b = 0
b = 0
b = 0
b in (-oo:0) U (0:+oo)
4 = 300/b // - 300/b
4-(300/b) = 0
4-300*b^-1 = 0
-300*b^-1 = -4 // : -300
b^-1 = 1/75
-1 < 0
1/(b^1) = 1/75 // * b^1
1 = 1/75*b^1 // : 1/75
75 = b^1
b = 75
b = 75
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