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14 February, 08:54

Three consecutive terms of an arithmetic sequence have a sum of 12 and a product of - 80. Find the terms. Hint: let the terms be x-d, x and x+d.

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  1. 14 February, 09:01
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    (x - d) + x + (x + d) = 12 - - > Create an equation using the first piece of information - "Three consecutive terms ... have a sum of 12"

    x - d + x + x + d = 12 - - > Simplify the left side of this equation (d cancels out)

    3x = 12 - - > Divide both sides by 3

    x = 4

    Use the value of x (x = 4) to find the value of d. To do this, set up another equation using the second piece of information.

    (x - d) * (x + d) * x = - 80 - - > "Three consecutive terms ... have ... a product of - 80". Then, substitute the value of x (4) into this equation.

    (4 - d) * (4 + d) * 4 = - 80 - - > Multiply out the sets of brackets, the * 4 is dealt with afterwards

    4 (16 - 4d + 4d - d²) = - 80 - - > Simplify the expression inside the brackets

    4 (16 - d²) = - 80 - - > Multiply out these brackets by the 4

    64 - 4d² = - 80 - - > Subtract 64 from both sides

    - 4d² = - 144 - - > Divide both sides by - 4

    d² = 36 - - > Square root both sides

    d = 6

    Now, find the values of the terms of the sequence by using substituting the values of x and d into the expressions given.

    1. x - d = 4 - 6 = - 2

    2. x = 4

    3. x + d = 4 + 6 = 10

    The three terms are - 2, 4, 10.
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