1) x^4-x^3-67x^2-11x+6=
=x^4-9x^3+8x^3+2x^2-72x^2+3x^2+16x-27x+6=
=(x^4-9x^3+2x^2)+(8x^3-72x^2+16x)+(3x^2-27x+6)=
=x^2(x^2-9x+2)+8x(x^2-9x+2)+3(x^2-9x+2)=
=(x^2+8x+3)(x^2-9x+2)
2) x^4+6x^3-21x^2+78x-16=
= x^4-3x^3+9x^3+8x^2-27x^2-2x^2+72x+6x-16=
=(x^4-3x^3+8x^2)+(9x^3-27x^2+72x)-(2x^2-6x+16)=
=x^2(x^2-3x+8)+9x(x^2-3x+8)-2(x^2-3x+8)=
=(x^2+9x-2)(x^2-3x+8)
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3) 4x^4-3x^3-8x^2+3x^3+4=
=4x^4-8x^2+4=
=4(x^4-x^2-x^2+1)=
=((4(x^2(x^2-1)-(x^2-1))=
=4((x^2-1)(x^2-1))=
=4(x^2-1)^2=
=4(x-1)^2(x+1)^2