Selecting the most crucial part of Cengage Mathematics depends on your specific strengths and weaknesses in the subject. However, considering the general importance of different topics for JEE preparation, here's a breakdown:

• Algebra: This part is foundational and covers a wide range of topics essential for problem-solving. It's generally considered a must-have.
• Calculus: Another core area, calculus is vital for both JEE Main and Advanced. It's highly recommended to have this part.
• Coordinate Geometry: This part is crucial for understanding geometric interpretations of algebraic equations and is essential for problem-solving.
• Trigonometry: While important, trigonometry might be covered to a certain extent in other parts. Assess your proficiency in this area before deciding.
• Vector and 3D Geometry: This part is crucial for physics as well as mathematics. It's generally recommended.

If you have to choose only one or two parts, prioritize Algebra and Calculus. However, if you feel confident in these areas, consider adding Coordinate Geometry or Vector and 3D Geometry based on your needs.

Remember, the best approach is to analyze your strengths and weaknesses and choose the part(s) that address your specific requirements.

hope this helps you!!