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Mathematics

Using the Remainder Theorem, factorise the expression 3x3 + 10x2 + x - 6. Hence, solve the equation 3x3 + 10x2 + x - 6 = 0.

Factorisation

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Answer

For x = -1, the value of 3x3 + 10x2 + x - 6,

= 3(-1)3 + 10(-1)2 + (-1) - 6

= 3(-1) + 10(1) - 7

= 10 - 10

= 0.

Hence, (x + 1) is the factor of 3x3 + 10x2 + x - 6.

On dividing 3x3 + 10x2 + x - 6 by (x + 1),

x+1)3x2+7x6x+1)3x3+10x2+x6x+13x3+3x2x+13x3+107x2+xx+13x3+17x2+7xx+13x3+1+2x26x6x+12x3++2x241 +6x+6x+12x3++2x24x×\begin{array}{l} \phantom{x + 1)}{3x^2 + 7x - 6} \ x + 1\overline{\smash{\big)}3x^3 + 10x^2 + x - 6} \ \phantom{x + 1}\underline{\underset{-}{}3x^3 \underset{-}{+} 3x^2} \ \phantom{{x + 1}3x^3+10}7x^2 + x \ \phantom{{x + 1}3x^3+1}\underline{\underset{-}{}7x^2 \underset{-}{+} 7x} \ \phantom{{x + 1}{3x^3+1}{+2x^2}}-6x - 6 \ \phantom{{x + 1}{2x^3+}{+2x^2}{41\space}}\underline{\underset{+}{-}6x \underset{+}{-} 6} \ \phantom{{x + 1}{2x^3+}{+2x^2-}{-4x}}\times \end{array}

we get, quotient = 3x2 + 7x - 6

Factorising 3x2 + 7x - 6,

= 3x2 + 9x - 2x - 6

= 3x(x + 3) - 2(x + 3)

= (3x - 2)(x + 3).

∴ 3x2 + 7x - 6 = (3x - 2)(x + 3).

Hence, 3x3 + 10x2 + x - 6 = (x + 1)(3x - 2)(x + 3).

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