<p>已知数列<img src="https://file.gaojiufeng.cn/learnAppQuestion/18/a7/18a782aa0f88e44458c6ca1f33fc7056.jpg" style="width: 100%;height: auto;">中<img src="https://file.gaojiufeng.cn/learnAppQuestion/66/18/66188efe3a38e36b2e3f26322c2b1e4b.jpg" style="width: 100%;height: auto;">前<img src="https://file.gaojiufeng.cn/learnAppQuestion/87/b6/87b69314e468b04c8743115fa67f8b0a.jpg" style="width: 100%;height: auto;">项和为<img src="https://file.gaojiufeng.cn/learnAppQuestion/a9/08/a908076470d4016fdd623100a84229f1.jpg" style="width: 100%;height: auto;">且<img src="https://file.gaojiufeng.cn/learnAppQuestion/63/b9/63b970fcd199d1260a65f1adead20faf.jpg" style="width: 100%;height: auto;">若数列<img src="https://file.gaojiufeng.cn/learnAppQuestion/b5/7a/b57abfaad508be0799980805979a9638.jpg" style="width: 100%;height: auto;">满足<img src="https://file.gaojiufeng.cn/learnAppQuestion/c3/a8/c3a86e12d04fe7c1e5f3157e07631a59.jpg" style="width: 100%;height: auto;">为数列<img src="https://file.gaojiufeng.cn/learnAppQuestion/b5/7a/b57abfaad508be0799980805979a9638.jpg" style="width: 100%;height: auto;">的前<img src="https://file.gaojiufeng.cn/learnAppQuestion/87/b6/87b69314e468b04c8743115fa67f8b0a.jpg" style="width: 100%;height: auto;">项和，求<img src="https://file.gaojiufeng.cn/learnAppQuestion/f4/06/f40667c634ff30f2fb183178038486f1.jpg" style="width: 100%;height: auto;">的表达式.</p>