<p>已知<img src="https://file.gaojiufeng.cn/learnAppQuestion/72/37/7237aab053a3bf86edc031e867e9ceb3.jpg" style="width: 100%;height: auto;">是等差数列，且<img src="https://file.gaojiufeng.cn/learnAppQuestion/3a/60/3a604a63871078d903db1cdf9d9c1fed.jpg" style="width: 100%;height: auto;"><br>求<img src="https://file.gaojiufeng.cn/learnAppQuestion/72/37/7237aab053a3bf86edc031e867e9ceb3.jpg" style="width: 100%;height: auto;">的前<img src="https://file.gaojiufeng.cn/learnAppQuestion/0d/a2/0da2e5116c3beb8def21be33f248a56c.jpg" style="width: 100%;height: auto;">项和<img src="https://file.gaojiufeng.cn/learnAppQuestion/82/ca/82ca714a350f642c434aa1a1ea7e83eb.jpg" style="width: 100%;height: auto;"></p>