A(x)+B(x)=3$x^4$-$\frac{3}{4}$.x³+2x²-3+8$x^4$+$\frac{1}{5}$.x³-9x+$\frac{2}{5}$
=11$x^4$-$\frac{11}{20}$.x³+2x²-9x-$\frac{13}{5}$
A(x)-B(x)=3$x^4$ - $\frac{3}{4}$.x³+ 2x² - 3-(8$x^4$ + $\frac{1}{5}$.x³ - 9x + $\frac{2}{5}$ )
=3$x^4$ - $\frac{3}{4}$ x³ + 2x² - 3-8$x^4$ - $\frac{1}{5}$ x³ + 9x - $\frac{2}{5}$
= -5$x^4$-$\frac{19}{20}$.x³+2x²+9x-$\frac{17}{5}$
B(x)-A(x)= 8$x^4$+ $\frac{1}{5}$.x³ - 9x + $\frac{2}{5}$ -(3$x^4$ - $\frac{3}{4}$ x³ + 2x² - 3)
=8$x^4$ + $\frac{1}{5}$ .x³ - 9x + $\frac{2}{5}$ -3$x^4$+$\frac{3}{4}$ x³ -2x² + 3
=5$x^4$+$\frac{19}{20}$.x³-2x²-9x+$\frac{17}{5}$