y'=15x^4-20x^3-30x^2+60x-5
y''=60x^3-60x^2-60x+60
=60(x+1)(x-1)^2
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y'=15x^4-20x^3-30x^2+60x-5
y''=60x^3-60x^2-60x+60
=60(x^3-x^2-x+1)
=60[x^2(x-1)-(x-1)]
=60(x-1)(x^2-1)
=60(x+1)(x-1)^2