Sum: 1 + 45 + 752 + +3n−25n−1．

Let Sn = 1 + 45 + 752 + +Then 15sn = 15 + 452 + 753 + +3N − 55n − 1 + 3N − 25N & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; ② ① - ②: 45sn = 1 + 35 + 352 + +35n−1−3n−25n=1+3×15(1−15n−1)1−15−3n−25n=7×5n−12n−74×5n∴Sn=7×5n−12n−716×5n−1

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