Maxwell's equal-area law with several pairs of conjugate variables for RN-AdS black holes Xiong-Ying Guo, Huai-Fan Li and Ren Zhao Eur. Phys. J. Plus, 134 6 (2019) 277 Published online: 20 June 2019 DOI: 10.1140/epjp/i2019-12790-y