Methods A arithmetic progression and a geometric progression were multiplied to form a series, which was solved by dislocation subtraction.

When summing by dislocation subtraction, attention should be paid to:

(1) When writing the expressions of "Sn" and "qSn", we should pay special attention to the "alignment errors" of the two expressions.

(2) the sequence {cn} satisfies: Cn=(an+b)q(a≠0, q≠0, g≠ 1),

Then the sum Sn of the first n terms satisfies: s = (an+b) q "+c.

If you don't understand the direct formula, you should pay attention to the fact that this problem is the n power of 3, not the n- 1 power of 3, so you should change it.