This study investigated student’s levels of theoretical and deductive reasoning patterns and achievement in plane geometry. The study adopted casual comparative research design. A total of 368 mathematics students were selected for the study involving multi-stage sampling procedure. Plane Geometry Achievement Test (PGAT) and Plane Geometry Reasoning Pattern Classification Test (PGRPCT) were instruments used for data collection. The instruments were subjected to both face validation and content validation. The reliability of the PGAT was determined using Kudder-Richardson (K-R20) method and reliability index of 0.81 was obtained while PGRPCT was ascertained using Pearson Product Moment Correlation Coefficient and reliability coefficient of 0.88 was obtained. The result of the study revealed that mathematics students employed both theoretical and deductive reasoning patterns while solving geometric problems. There was a significant difference in the mean achievement scores of students in the two levels of reasoning patterns. Students need preliminary explorations of the properties of a geometric shape before they can attempt to write a definition of it.

Achievement test, classification test, geometry reasoning patterns, theoretical, deductive reasoning, mathematics