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Mathematics

If (3x - 2) is a factor of 3x3 - kx2 + 21x - 10, find the value of k.

Factorisation

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Answer

f(x) = 3x3 - kx2 + 21x - 10

If, (3x - 2) or 3(x(23))3(x - \big(\dfrac{2}{3}\big)) is a factor of f(x) then f(23\dfrac{2}{3}) = 0

3(23)3k(23)2+21(23)10=03(827)k(49)+1410=0894k9+4=084k+369=0444k=04k=44k=11.\therefore 3\big(\dfrac{2}{3}\big)^3 - k\big(\dfrac{2}{3}\big)^2 + 21\big(\dfrac{2}{3}\big) - 10 = 0 \\[1em] \Rightarrow 3\big(\dfrac{8}{27}\big) - k\big(\dfrac{4}{9}\big) + 14 - 10 = 0 \\[1em] \Rightarrow \dfrac{8}{9} - \dfrac{4k}{9} + 4 = 0 \\[1em] \Rightarrow \dfrac{8 - 4k + 36}{9} = 0 \\[1em] \Rightarrow 44 - 4k = 0 \\[1em] \Rightarrow 4k = 44 \\[1em] k = 11.

Hence, the value of k is 11.

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