Question

Lapid.

(ii) Find the remainder when f(x) is
divided by (x - 1)(x + 2).
14) The remainder when the expression
ax' + bx + 2x + c is divided by x-1
is twice of that when it is divided by
x + 1. Show that c = 3a - b + 6.
​

Answer 1

ii) When a polynomial f(x) is divided by (x-1) and (x+2) it leaves remainder 5 and 17 respectively. find the remainder when f(x) is divided by (x-1) (x+2)

14) The remainder when the expression ax³ + bx² + 2x + c is divided by x-1

is twice of that when it is divided by x + 1. Show that c = 3a - b + 6.

Answer:

ii) R(x) = -4x + 9

14) c = 3a - b + 6 ( Proved)

Explanation:

14) The correct expression is
`ax^3 + bx^2 + 2x + c`

To get the remainder when the expression
`ax^3 + bx^2 + 2x + c` is divided by

x - 1, let x - 1 = 0; x = 1

Remainder:

`R_1(x) = a(1)^3 + b(1)^2 + 2(1) + c\\R_1(x) = a + b + 2 + c`

When
`ax^3 + bx^2 + 2x + c` is divided by x + 1

Let x + 1 = 0; x = -1

Remainder:

`R_2(x) = a(-1)^3 + b(-1)^2 + 2(-1) + c\\R_1(x) = -a + b - 2 + c`

According to the question, R₁(x) = 2R₂(x)

a + b + 2 + c = 2(-a + b - 2 + c)

a + 2a +b - 2b + 2 + 4 = 2c - c

c = 3a - b + 6 ( Proved)

ii)

The dividend is f(x)

(x - 1)(x + 2) is the divisor, i.e. D(x) = (x - 1)(x + 2)

Let the quotient = A(x)

Let the Remainder, R(x) = ax + b..............(1)

Therefore, f(x) = A(x)D(x) + R(x)

f(x) = A(x)(x - 1)(x + 2) + R(x)...................(2)

When f(x) is divided by x - 1, x = 1

Put x = 1 into equation (2) knowing that R(1) = 5

f(1) = R(1) = 5

R(1) = a(1) + b = 5

a + b = 5....................(3)

When f(x) is divided by x + 2, x = -2

Put x = -2 into equation (2) knowing that R(-2) = 17

f(-2) = R(-2) = 17

R(-2) = a(-2) + b = 17

-2a + b = 17..................(4)

Subtracting equation (1) from (2)

-3a = 12

a = -12/3

a = -4

Substitute the value of "a" into equation (4)

-2(-4) + b = 17

8 + b = 17

b = 9

Since R(x) = ax + b

R(x) = -4x + 9