*Lời giải :
`a)`
\(\left\{ \begin{array}{l}A(x)=-3+2x^3-4x^2+5x^3-7x^4\\B (x) = 4x^2 - 3x^4 + 9x^2 - 5x\\C(x)=-3x+2-7x+8x^4-3x^2\end{array} \right.\)
`=` \(\left\{ \begin{array}{l}A(x)=-3 + 7x^3 - 4x^2 - 7x^4 \\B(x)=13x^2 - 3x^4 - 5x\\C(x) = -9x + 2 +8x^4-3x^2\end{array} \right.\)
Sắp xếp \(\left\{ \begin{array}{l}A(x)\\B(x)\\C(x)\end{array} \right.\) theo lũy thừa tăng dần :
\(\left\{ \begin{array}{l}A(x)=-7x^4+7x^3-4x^2-3\\B(x)=-3x^4 + 13x^2 - 5x\\C(x) = 8x^4 - 3x^2 -9x + 2\end{array} \right.\)
`b)`
`A (x) + B (x) = -7x^4 + 7x^3 - 4x^2 - 3 - 3x^4 + 13x^2 - 5x`
`-> A(x) + B (x) = (-7x^4 - 3x^4) + 7x^3 + (-4x^2 + 13x^2) - 3 - 5x`
`-> A (x) + B (x) = -10x^4 + 7x^3 + 9x^2 - 3 - 5x`
`c) B (x) + C (x) = -3x^4 + 13x^2 - 5x + 8x^4 - 3x^2- 9x + 2`
`-> B (x) + C (x) = (-3x^4 + 8x^4) + (13x^2 - 3x^2) + (-5x - 9x) + 2`
`-> B (x) + C (x)=5x^4 + 10x^2 - 14x + 2`
`d) A (x) - C (x) = -7x^4+7x^3-4x^2-3 - 8x^4 + 3x^2 + 9x - 2`
`-> A (x) - C(x) = (-7x^4 - 8x^4) + 7x^3 + (-4x^2 + 3x^2) + (-3 - 2) + 9x`
`-> A (x) - C (x) = -15x^4 + 7x^3 + x^2 - 5 + 9x`
`e) A (x) + B (x) + C (x) = -7x^4+7x^3-4x^2-3-3x^4 + 13x^2 - 5x + 8x^4 - 3x^2 -9x + 2`
`-> A (x) + B (x) + C (x) = (-7x^4 - 3x^4 + 8x^4) + 7x^3 + (-4x^2 + 13x^2 - 3x^2) + (-5x - 9x) + (-3 + 2)`
`-> A (x) + B (x) + C (x) = -2x^4 + 7x^3 + 6x^2 - 14x - 1`
