SEQUENCE AND SERIES QUESTION.

Please explain the highlighted step in the following solution:
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SEQUENCE AND SERIES QUESTION. Please explain the highlighted step in the following solution:​ QUESTION Let al , , a3 . , all be real numbers satisfying al = 15 , 27 — > O and ak = 2ak_1 - for 90, then the value of 11 11 If al 4-a2+... 4-011 11 is equal to. D