Câu 3:
a, A(x)= 5$x^{4}$-6$x^{2}$-2$x^{3}$+7x+6
→ 5$x^{4}$-2$x^{3}$-6$x^{2}$+7x+6
B(x)= -7x+3$x^{2}$+2$x^{3}$-5
→ 2$x^{3}$+3$x^{2}$-7x-5
b,
A(x)+B(x)=5$x^{4}$-2$x^{3}$-6$x^{2}$+7x+6+2$x^{3}$+3$x^{2}$-7x-5
= 5$x^{4}$-3$x^{2}$+1
A(x)-B(x)= (5$x^{4}$-2$x^{3}$-6$x^{2}$+7x+6)-(2$x^{3}$+3$x^{2}$-7x-5)
= 5$x^{4}$-2$x^{3}$-6$x^{2}$+7x+6-2$x^{3}$-3$x^{2}$+7x+5
= 5$x^{4}$-4$x^{3}$-9$x^{2}$+14x
