When the expression x3+2kx+2x^{3}+2kx+2 is divided by x+2x+2, the remainder is 3 less than the remainder obtained when the expression is divided by $x-1.\nFind the value of k. Solve the equation 2x3−5x2−3x+10=02x^{3}-5x^{2}-3x+10=0, expressing non-integer roots in exact form.

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Worked solution

5(a)

1. Find the remainder when the expression is divided by $x+2$.

Let f(x)=x3+2kx+2f(x) = x^{3}+2kx+2.
By the Remainder Theorem, the remainder when divided by x+2x+2 is f(−2)f(-2).
f(−2)=(−2)3+2k(−2)+2f(-2) = (-2)^{3} + 2k(-2) + 2
=−8−4k+2= -8 - 4k + 2
=−6−4k= -6 - 4k

2. Find the remainder when the expression is divided by $x-1$.

The remainder when divided by x−1x-1 is f(1)f(1).
f(1)=(1)3+2k(1)+2f(1) = (1)^{3} + 2k(1) + 2
=1+2k+2= 1 + 2k + 2
=3+2k= 3 + 2k

3. Form an equation using the given condition and solve for $k$.

The remainder for x+2x+2 is 3 less than the remainder for x−1x-1:
f(−2)=f(1)−3f(-2) = f(1) - 3
−6−4k=(3+2k)−3-6 - 4k = (3 + 2k) - 3
−6−4k=2k-6 - 4k = 2k
6k=−66k = -6
k=−1k = -1

Answer

k=−1k = -1

5(b)

1. Use the Factor Theorem to find a linear factor of the cubic polynomial.

Let f(x)=2x3−5x2−3x+10f(x) = 2x^3 - 5x^2 - 3x + 10.
f(1)=2(1)3−5(1)2−3(1)+10=4≠0f(1) = 2(1)^3 - 5(1)^2 - 3(1) + 10 = 4 \neq 0
f(−1)=2(−1)3−5(−1)2−3(−1)+10=6≠0f(-1) = 2(-1)^3 - 5(-1)^2 - 3(-1) + 10 = 6 \neq 0
f(2)=2(2)3−5(2)2−3(2)+10=16−20−6+10=0f(2) = 2(2)^3 - 5(2)^2 - 3(2) + 10 = 16 - 20 - 6 + 10 = 0
Since f(2)=0f(2) = 0, (x−2)(x - 2) is a factor.

2. Find the quadratic factor by comparing coefficients or long division.

Let 2x3−5x2−3x+10=(x−2)(ax2+bx+c)2x^3 - 5x^2 - 3x + 10 = (x - 2)(ax^2 + bx + c).
By observation, a=2a = 2 and c=−5c = -5.
So, 2x3−5x2−3x+10=(x−2)(2x2+bx−5)2x^3 - 5x^2 - 3x + 10 = (x - 2)(2x^2 + bx - 5).
Comparing the coefficients of x2x^2:
−4+b=−5  ⟹  b=−1-4 + b = -5 \implies b = -1
The quadratic factor is 2x2−x−52x^2 - x - 5.

3. Solve the quadratic equation to find the remaining roots.

2x2−x−5=02x^2 - x - 5 = 0
Using the quadratic formula:
x=−(−1)±(−1)2−4(2)(−5)2(2)x = \frac{-(-1) \pm \sqrt{(-1)^2 - 4(2)(-5)}}{2(2)}
x=1±1+404x = \frac{1 \pm \sqrt{1 + 40}}{4}
x=1±414x = \frac{1 \pm \sqrt{41}}{4}

Answer

x=2,1±414x = 2, \frac{1 \pm \sqrt{41}}{4}

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