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Mathematics

(x - 2) is a factor of the expression x3 + ax2 + bx + 6. When this expression is divided by (x - 3), it leaves the remainder 3. Find the values of a and b.

Factorisation

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Answer

Let f(x) = x3 + ax2 + bx + 6

Given, (x - 2) is factor of f(x), hence, f(2) = 0 by factor's theorem

23+a(2)2+b(2)+6=08+4a+2b+6=04a+2b+14=02a+b+7=0b=72a (Equation 1) \therefore 2^3 + a(2)^2 + b(2) + 6 = 0 \\[0.5em] \Rightarrow 8 + 4a + 2b + 6 = 0 \\[0.5em] \Rightarrow 4a + 2b + 14 = 0 \\[0.5em] \Rightarrow 2a + b + 7 = 0 \\[0.5em] b = -7 - 2a \text{ (Equation 1) }

Given, on dividing f(x) by (x - 3) remainder left is 3

By remainder theorem, remainder = f(3)

33+a(3)2+b(3)+6=327+9a+3b+6=333+9a+3b=3\therefore 3^3 + a(3)^2 + b(3) + 6 = 3 \\[0.5em] \Rightarrow 27 + 9a + 3b + 6 = 3 \\[0.5em] \Rightarrow 33 + 9a + 3b = 3

On dividing equation by 3,

11+3a+b=1\Rightarrow 11 + 3a + b = 1

Putting value of b from equation 1,

11+3a72a=1a+4=1a=3 and b=72a=72(3)=7+6=1\Rightarrow 11 + 3a - 7 - 2a = 1 \\[0.5em] \Rightarrow a + 4 = 1 \\[0.5em] \Rightarrow a = -3 \\[0.5em] \text{ and } b = -7 - 2a = -7 - 2(-3) = -7 + 6 = -1

Hence, the value of a = -3 and b = -1.

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