Mathematical creative ability is one of the most important skills students must have to process the information provided in resolving the problem. Before using mathematical creative skills, prior knowledge becomes the most crucial thing that allows students to connect all existing information so that they can construct new knowledge through assimilation or accommodation processes. The process of forming mathematical concepts with metacognitive questions that might be carried out by students causes a metacognitive process in students that will affect their mathematical behavior. The purpose of this study is to (1) analyze prior knowledge of what students miss or forget so that they have difficulty to answer the given geometry problem, (2) how the learning path of creative thinking of students with the application of metacognitive approach. This type of research is Design Research to improve the quality of learning. This type of research is research design, data collection techniques .The researcher gave 2 geometry questions to 38 8th graders selected randomly in SMP Medan city. Questions given are tailored to Cognitive level 4 (C4) for questions 1 and C5 for question 2 based on Bloom's taxonomy. Data analysis techniques are descriptive qualitative.This study shows that prior knowledge becomes important to build students' mathematical creative ability to gain new knowledge, especially in the field of geometry. The most problematic topics that make it difficult for them to understand geometry are the area of the rectangle and the cube webs. In dividing the rectangle into two equal parts, students still have not created another form of flat build or have not been able to get out of the rectangular pattern or exactly the same as the available problem. There are five phases of learning trajectory of hierarchically creative mathematical thinking, which is orientation to problem, problem solving plan, plan realization, previous knowledge mastery / concept of mathematical creativity and evaluation of result obtained. Students do metacognition on the learning path of creative thinking in a comprehensive way from evaluation to planning, action to the formation of prior knowledge and selection of creative ideas. From these explanations, it is important that teachers need to ensure students have enough prior knowledge to make it easier to construct new knowledge, as well as to make learning fun and meaningful so that students will remember knowledge in long-term memory.

Prior knowledge becomes the most crucial thing that allows students to connect all existing information so that they can construct new knowledge through assimilation or accommodation processes. The aims of this research is to (1) analyze students' initial knowledge of what they miss or forget so they have difficulty answering the given geometry, (2) know the impact of Hypothetical Learning Trajectory (HLT) or learning path of creative thinking of students with the application of metacognitive approach . This type of research is Design Research in term of improving the quality of learning related to understanding the concept of Geometry. This study shows that prior knowledge becomes important to build students' mathematical creative ability to gain new knowledge of geometry. The most interesting topic is the area of the rectangle. The results of the study found that (1) the students were not creative yet or not able to get out of the existing flat pattern. Students are inseparable from the shape and drawing of the flat figure which is exactly the same as the available question, (2) there are five phases of learning path of hierarchical creative thinking of mathematics, that is orientation to problem, problem solving plan, plan realization, mastery of previous knowledge / concept of creativity mathematics and evaluation of the results obtained.