6. The first three terms of a geometric sequence are
$$u _ { 1 } = 3 k + 4 \quad u _ { 2 } = 12 - 3 k \quad u _ { 3 } = k + 16$$
where \(k\) is a constant.
Given that the sequence converges,
- Find the value of \(k\), giving a reason for your answer.
- Find the value of \(\sum _ { r = 2 } ^ { \infty } u _ { r }\)
[0pt]
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