juantheron
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The third term wouldn't be 27x, it would be 81x.It's fairly obvious it has something to do with p(x)= x^3 -15x^2 +27x +d
We require the above to have three real solutions, which means it's derivative must have two real solutions (for a cubic to cut the x axis three times, it must turn around twice)
p ' (x) = 3x^2 -30x +27 =3x^2 -3x -27x +27 =0
3x(x-1)-27(x-1)=0
(3x-27)(x-1) =0
x= 9 or x=1
Continue from that somehow. I caqn't be bothered to finish it now.