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By (long) division:

. . . . . . . . . . .2x2 - 7x . + 2

. . . . . ----------------------

x + 2 | 2x3 - 3x2 - 12x + 4

. . . . . .2x3 + 4x2

. . . . . .-----------

. . . . . . . . . - 7x2 - 12x

. . . . . . . . . - 7x2 - 14x

. . . . . . . . . ------------

. . . . . . . . . . . . . . . . 2x + 4

. . . . . . . . . . . . . . . . 2x + 4

. . . . . . . . . . . . . . . . -------

. . . . . . . . . . . . . . . . . . . . .0

. . . . . . . . . . . . . . . . ====

(the "dot-spaces" are used to hold the characters in the right place of the division - they should be treated as blank)

Thus since

(x + 2)(2x2 - 7x + 2) = 2x3 - 3x2 - 12x + 4

(x + 2) is a factor of 2x3 - 3x2 - 12x + 4

In the division:

  1. the first term of the divisor (x) is compared with the highest power of x remaining in the dividend to find the next term of the quotient;
  2. the whole divisor is multiplied by this;
  3. then subtracted from the dividend;
  4. Steps 1-3 are repeated until there is no first term of the divisor (x) in the dividend.
  5. If the dividend is '0' (ie the last multiplication resulted in what was remaining in the dividend) then the divisor is a factor of the original dividend); otherwise it is not a factor.
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Q: How would you prove that x plus 2 is a factor of the expression 2x cubed -3x squared -12x plus 4?
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