To address the data shortage in the field of handwritten mathematical expression recognition (HMER), this paper proposes a novel method for integrating large-scale LaTeX rendered equations with limited handwritten equations. We develop a scalable data engine for large-scale LaTeX equation generation and build Tex80M, the largest equation dataset to date, comprising over 80 million high-quality training instances. Building on this, we propose TexTeller, the first large-scale HMER model, by hybrid training with Tex80M and the relatively small HME dataset. TexTeller achieves state-of-the-art (SOTA) performance on nearly all benchmarks. We make the model, dataset, and codebase publicly available to support further research.