Math Problem Statement

Given Q(x) = ax^3 - bx^2 - cx + 6. If (x + 2) is a factor of Q(x) and Q(x) passes through the points (-1,10) and (2,4), find the values of a, b and c. Hence, factorise Q(x) completely

Solution

Given the polynomial Q(x)=ax3bx2cx+6Q(x) = ax^3 - bx^2 - cx + 6, we are provided with the following information:

  1. (x+2)(x + 2) is a factor of Q(x)Q(x).
  2. Q(x)Q(x) passes through the points (1,10)(-1, 10) and (2,4)(2, 4).

Step 1: Use the fact that (x+2)(x + 2) is a factor.

Since (x+2)(x + 2) is a factor of Q(x)Q(x), by the Factor Theorem, we know: Q(2)=0Q(-2) = 0 Substitute x=2x = -2 into the polynomial: Q(2)=a(2)3b(2)2c(2)+6=0Q(-2) = a(-2)^3 - b(-2)^2 - c(-2) + 6 = 0 This simplifies to: 8a4b+2c+6=0(Equation 1)-8a - 4b + 2c + 6 = 0 \quad \text{(Equation 1)}

Step 2: Use the fact that Q(1)=10Q(-1) = 10.

Substitute x=1x = -1 into the polynomial: Q(1)=a(1)3b(1)2c(1)+6=10Q(-1) = a(-1)^3 - b(-1)^2 - c(-1) + 6 = 10 This simplifies to: ab+c+6=10-a - b + c + 6 = 10 ab+c=4(Equation 2)-a - b + c = 4 \quad \text{(Equation 2)}

Step 3: Use the fact that Q(2)=4Q(2) = 4.

Substitute x=2x = 2 into the polynomial: Q(2)=a(2)3b(2)2c(2)+6=4Q(2) = a(2)^3 - b(2)^2 - c(2) + 6 = 4 This simplifies to: 8a4b2c+6=48a - 4b - 2c + 6 = 4 8a4b2c=2(Equation 3)8a - 4b - 2c = -2 \quad \text{(Equation 3)}

Step 4: Solve the system of equations.

We now have the following system of linear equations:

  1. 8a4b+2c+6=0-8a - 4b + 2c + 6 = 0 8a4b+2c=6\Rightarrow -8a - 4b + 2c = -6
  2. ab+c=4-a - b + c = 4
  3. 8a4b2c=28a - 4b - 2c = -2

Step 4.1: Simplify the equations:

Equation 1: 8a4b+2c=6-8a - 4b + 2c = -6

Dividing by 2: 4a2b+c=3(Equation 1 simplified)-4a - 2b + c = -3 \quad \text{(Equation 1 simplified)}

Equation 3: 8a4b2c=28a - 4b - 2c = -2

Dividing by 2: 4a2bc=1(Equation 3 simplified)4a - 2b - c = -1 \quad \text{(Equation 3 simplified)}

Step 4.2: Add the simplified Equation 1 and Equation 3:

(4a2b+c)+(4a2bc)=3+(1)(-4a - 2b + c) + (4a - 2b - c) = -3 + (-1) Simplifying gives: 4b=4b=1-4b = -4 \Rightarrow b = 1

Step 4.3: Substitute b=1b = 1 into Equation 2:

a1+c=4a+c=5(Equation 2 simplified)-a - 1 + c = 4 \Rightarrow -a + c = 5 \quad \text{(Equation 2 simplified)} So, c=a+5c = a + 5.

Step 4.4: Substitute b=1b = 1 and c=a+5c = a + 5 into the simplified Equation 1:

4a2(1)+(a+5)=3-4a - 2(1) + (a + 5) = -3 Simplifying gives: 4a2+a+5=33a+3=3-4a - 2 + a + 5 = -3 \Rightarrow -3a + 3 = -3 This simplifies to: 3a=6a=2-3a = -6 \Rightarrow a = 2

Now, substitute a=2a = 2 back into the equations to find cc: c=2+5=7c = 2 + 5 = 7

Thus, the values are: a=2,b=1,c=7a = 2, \quad b = 1, \quad c = 7

Step 5: Factorize Q(x)Q(x) completely.

Substitute a=2a = 2, b=1b = 1, and c=7c = 7 into the original polynomial: Q(x)=2x3x27x+6Q(x) = 2x^3 - x^2 - 7x + 6

Given (x+2)(x + 2) is a factor: Q(x)=(x+2)(2x25x+3)Q(x) = (x + 2)(2x^2 - 5x + 3)

Now, factorize the quadratic 2x25x+32x^2 - 5x + 3: The quadratic can be factorized as: 2x25x+3=(2x3)(x1)2x^2 - 5x + 3 = (2x - 3)(x - 1)

Thus, the complete factorization of Q(x)Q(x) is: Q(x)=(x+2)(2x3)(x1)Q(x) = (x + 2)(2x - 3)(x - 1)

Final Answer:

  • Values: a=2a = 2, b=1b = 1, c=7c = 7
  • Factorization: Q(x)=(x+2)(2x3)(x1)Q(x) = (x + 2)(2x - 3)(x - 1)

Would you like a more detailed explanation on any step? Here are some related questions:

  1. How do you apply the Factor Theorem to factorize polynomials?
  2. What are the steps to solve a system of linear equations?
  3. How can you verify the factorization of a cubic polynomial?
  4. What techniques are there for factoring quadratic equations?
  5. How do transformations affect the graph of a polynomial function?

Tip: When factorizing polynomials, always check if simpler factors (like binomials) can be factored out first before attempting more complex methods.

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Math Problem Analysis

Mathematical Concepts

Polynomials
Factorization
Quadratic Equations
System of Equations

Formulas

Factor Theorem

Theorems

Factor Theorem

Suitable Grade Level

Grades 10-12