Question

2(1-k), k-1, k+8 are the first three terms of a geometrical sequence. Find the value of k.

Answer 1

If these are terms of a geometric sequence, they have a common ratio. That is, ...

... (k -1)/(2(1 -k)) = (k +8)/(k -1)

... (k -1)² = 2(1 -k)(k +8) . . . . . multiply by the product of the denominators.

... k² -2k +1 = -2k² -14k +16 . . . eliminate parentheses

... 3k² +12k -15 = 0 . . . . . . . . put in standard form (subtract the right side)

... 3(k +5)(k -1) = 0 . . . . . . . . . factor

Possible values of k are ... -5, +1. The solution k=1 is extraneous, as it makes the first two terms 0 and the third term 8. (It doesn't work.)

The value of k is -5.

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The three terms are 12, -6, 3. The common ratio is -1/2.