`c.`

`C(x) = (x^4 - 6x^3 + 2x^2 - x + 1) + (-x^4 + 10x^3 - 2x^2 + 3x + 3)`

`C(x) = (x^4 - x^4) + (-6x^3 + 10x^3) + (2x^2 - 2x^2) + (-x + 3x) + (1 + 3)`

`C(x) = 4x^3 + 2x + 4`

_________

`D(x) = (x^4 - 6x^3 + 2x^2 - x + 1) - (-x^4 + 10x^3 - 2x^2 + 3x + 3)`

`D(x) = x^4 - 6x^3 + 2x^2 - x + 1 + x^4 - 10x^3 + 2x^2 - 3x - 3`

`D(x) = (x^4 + x^4) + (-6x^3 - 10x^3) + (2x^2 + 2x^2) + (-x - 3x) + (1 - 3)`

`D(x) = 2x^4 - 16x^3 + 4x^2 - 4x - 2`

`d.`

`2x^3 + x + 2 = 0`

`x^3 + \frac{1}{2}x + 1 = 0`

Áp dụng công thức Cardano:

$x = \sqrt[3]{-\dfrac{q}{2} + \sqrt{\dfrac{q^2}{4} + \dfrac{p^3}{27}}} + \sqrt[3]{-\dfrac{q}{2} - \sqrt{\dfrac{q^2}{4} + \dfrac{p^3}{27}}}$

$\Delta = \dfrac{q^2}{4} + \dfrac{p^3}{27} = \dfrac{1^2}{4} + \dfrac{(\dfrac{1}{2})^3}{27} = \dfrac{1}{4} + \dfrac{1}{216}$

$x = \sqrt[3]{-\dfrac{1}{2} + \sqrt{\dfrac{55}{216}}} + \sqrt[3]{-\dfrac{1}{2} - \sqrt{\dfrac{55}{216}}}$

`sqrt(55/216) ~~ 0,5046`

`x~~`$ \sqrt[3]{-0,5 + 0,5046} + \sqrt[3]{-0,5 - 0,5046}$

`x~~`$\sqrt[3]{0,0046} + \sqrt[3]{-1,0046}$

`x~~ - 0,835`