[填空题]
Let $f(x)=e^{3 \sin \left(\frac{\pi x}{4}\right)}$ , for x>0 .
The k th maximum point on the graph of f has x -coordinate $x_{k}$ , where $ k \in \mathbb{Z}^{+}$ .
1. Given that $x_{k+1}=x_{k}+d$ , find d .
2. Hence find the value of n such that $\sum_{k=1}^{n} x_{k}=992$ .
