LYX 错误 \triangleq 符号未定义

Question

当我尝试在 LYX 中预览我的文章时，出现以下错误：

f\left(x\right)\triangleq  
                               \lim_{\delta x\rightarrow\infty}\frac{f  \left(...
The control sequence at the end of the top line  
of your error message was never \def'ed. If you have  
misspelled it (e.g., `\hobx'), type `I' and the correct  
spelling (e.g., `I\hbox'). Otherwise just continue,  
and I'll forget about whatever was undefined.  

我相信它指的是\triangleq符号。
我该如何修复它？