Wednesday, April 18, 2012

What is the reminder when f(x)=x^5+x^4+1 is divided by g(x)=(x-1)^3 ?

Since the divisor is of 3rd order, the reminder has to be
of 2nd order.


r=ax^2 + bx +
c


We 'll write the reminder
theorem:


f=g*q + r, where q is the quotient of the
division.


We notice that x=1 is a multiple root of the
polynomial g.


f(1)=g(1)*q(1) +
r(1)


By substituting the value x=1 into all polynomials,
we'll obtain:


f(1)= 1^5 + 1^4 +
1


f(1)=1+1+1


f(1)=3


g(1)=
(1-1)^3=0


r(1)=a+b+c


So, we'll
have:


3=0*q(1) +
a+b+c


3=a+b+c


Because of the
fact that x=1 is a multiple root, it will cancel the first derivative of the expression:
f=g*c + r


f' =(g*c)' + r'


5x^4
+4 x^3=3*(x-1)^2*q+(x-1)^3*q' +2*ax+ b


By substituting the
value x=1 into all polynomials, we'll
obtain:


5+4=2*a+b


9=2*a+b


We'll
calculate the first derivative of the expression


5x^4 +4
x^3=3*(x-1)^2*q+(x-1)^3*q' +2*ax+ b


(5x^4 +4
x^3)'=(3*(x-1)^2*q+(x-1)^3*q' +2*ax+ b)'


5*4* x^3 + 4*3*
x^2=6*(x-1)*q+3*(x-1)^2*q'+3*(x-1)^2*q'+(x-1)^3*q"+2a


By
substituting the value x=1 into all polynomials, we'll
obtain:


20+12=2*a


We'll divide
by 2:


a = 10 + 6


a =
16


2a+b=9


2*16 + b =
9


b = 9 - 32


b =
-23


a+b+c =
3


16-23+c=3


-7+c=3


c=3+7


c=10


The
reminder is: 16x^2 - 23x + 10

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