P(x)=5x-8x³+15x²-6x+12= -8x³+15x²-x+12
Q(x)=13x+6x³+7x²+9x+13=6x³+7x²+22x+13
a) P(x)+Q(x)= -8x³+15x²-x+12+6x³+7x²+22x+13
= -2x³+22x²+21x+25
P(x)-Q(x)= -8x³+15x²-x+12- (6x³+7x²+22x+13)
= -8x³+15x²-x+12-6x³-7x²-22x-13
= -14x³+8x²-23x-1
b) thay x=1 vào P(x)
P(x)=-8.1³+15.1²-1+12=-8+15-1+12=18