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D( l )
l = 0
l = 0
l = 0
l in (-oo:0) U (0:+oo)
12/l = 14/3.5 // - 14/3.5
12/l-(14/3.5) = 0
12/l-4 = 0
12*l^-1 = 4 // : 12
l^-1 = 1/3
-1 < 0
1/(l^1) = 1/3 // * l^1
1 = 1/3*l^1 // : 1/3
3 = l^1
l = 3
l = 3
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