The most widely used math textbook for 9th grade in the United States is "Glencoe Mathematics: Applications and Connections." This textbook is known for its comprehensive coverage of algebra, geometry, and statistics, which aligns with the Common Core State Standards. It is commonly adopted in many schools across the country due to its alignment with the curriculum and its focus on real-world applications.

The popularity of "Glencoe Mathematics: Applications and Connections" can be attributed to several factors. Firstly, it is aligned with the Common Core State Standards, which have been adopted by a majority of states in the U.S. This alignment ensures that the content is relevant and prepares students for higher-level mathematics and future academic pursuits. Secondly, the textbook is well-structured and provides a clear progression of concepts, making it easier for students to follow along and understand the material. Additionally, the inclusion of real-world applications helps students see the relevance of mathematics in everyday life, which can enhance their engagement and motivation.

Data from the National Center for Education Statistics (NCES) indicates that "Glencoe Mathematics: Applications and Connections" is one of the most frequently used textbooks in 9th-grade mathematics classrooms. According to the 2015-2016 School Survey on Classroom Instruction, approximately 45% of 9th-grade mathematics teachers reported using this textbook. This widespread adoption is a testament to its effectiveness and popularity among educators.

Furthermore, the textbook's focus on problem-solving and critical thinking skills is in line with the evolving educational landscape in the U.S. As schools increasingly emphasize the development of these skills, "Glencoe Mathematics: Applications and Connections" stands out for its emphasis on these areas. The inclusion of various types of problems and exercises encourages students to think critically and apply their knowledge in different contexts.

In conclusion, "Glencoe Mathematics: Applications and Connections" is the predominant math textbook for 9th grade in the U.S. Its alignment with the Common Core State Standards, clear structure, focus on real-world applications, and emphasis on problem-solving and critical thinking contribute to its widespread adoption and effectiveness in the classroom.