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Mathematics

If (x - 2) is a factor of the expression 2x3 + ax2 + bx - 14 and when the expression is divided by (x - 3), it leaves a remainder 52, find the values of a and b.

Factorisation

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Answer

Let f(x) = 2x3 + ax2 + bx - 14

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

2(2)3+a(2)2+b(2)14=016+4a+2b14=02+4a+2b=0\therefore 2(2)^3 + a(2)^2 + b(2) - 14 = 0 \\[0.5em] \Rightarrow 16 + 4a + 2b - 14 = 0 \\[0.5em] \Rightarrow 2 + 4a + 2b = 0

On dividing equation by 2,

2a+b+1=0b=12a (Equation 1) \Rightarrow 2a + b + 1 = 0 \\[0.5em] \Rightarrow b = -1 - 2a \text{ (Equation 1) }

Given, on dividing f(x) by (x - 3) remainder left is 52

By remainder theorem, remainder = f(3)

2(3)3+a(3)2+b(3)14=5254+9a+3b14=529a+3b+40=529a+3b=123a+b=4\therefore 2(3)^3 + a(3)^2 + b(3) - 14 = 52 \\[0.5em] \Rightarrow 54 + 9a + 3b - 14 = 52 \\[0.5em] \Rightarrow 9a + 3b + 40 = 52 \\[0.5em] \Rightarrow 9a + 3b = 12 \\[0.5em] \Rightarrow 3a + b = 4

Putting value of b from equation 1,

3a12a=4a=5 and b=12a=110=11\Rightarrow 3a - 1 - 2a = 4 \\[0.5em] \Rightarrow a = 5 \\[0.5em] \text{ and } b = -1 - 2a = -1 - 10 = -11

Hence, the value of a = 5 and b = -11.

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