Based on a coding of 39 videotaped three-lesson units on the introduction to the Pythagorean Theorem, three teaching patterns were identified: lecturing, developing based on a problem, and discovery based on a problem. The analysis showed no effect of the teaching patterns on student achievement, whereas effects were discovered on students' perceived learning quality. The discovery teaching pattern had negative effects on the emotional quality of learning. However, this pattern exhibited high degree of externally rated cognitive activation. The lecturing approach to the introduction to the Pythagorean Theorem supported students' self-perceived understanding. (Contains 4 tables.)