Cita APA (7a ed.)

Helgren, D. M., & Sager, R. J. (1995). World geography today. Holt, Rinehart and Winston.

Cita Chicago Style (17a ed.)

Helgren, David M., y Robert J. Sager. World Geography Today. Austin, TX: Holt, Rinehart and Winston, 1995.

Cita MLA (9a ed.)

Helgren, David M., y Robert J. Sager. World Geography Today. Holt, Rinehart and Winston, 1995.