ABSTRAK The research aims to know the types and the causes of error in solving the problem of sine and cosine rules in terms of students' initial abilities. There are three ability groups namely high initial ability, medium initial ability, and low initial ability.
The research subjects were selected using purposive sampling. Research subject consists of  two high-initial ability students, two students with medium initial abilities, and two low-initial ability students. Data collection is done by using test and interview techniques. The analysis is done by Newman's method Error Analysis.
The results of the study are as follows. Students with high initial abilities
made an encoding error type. The reason is that students are not careful when re- check the answer. Students with medium initial abilities are doing transformation error type. The reason is student understanding about the concept of sine and cosine rules is less so that errors occur when they apply the formula. The next error is the process skills error. The cause is the ability of students to carry out numerical  operations  that  are  less.  One  of  the  students  with  medium  initial abilities is solving the fraction that will converted into multiplication, there is a tribe that is supposed to be the denominator but instead comes along multiplied. Another cause is the lack of mastery of the concept of sine and cosine rules. This error occurs when students with medium initial abilities are going to apply the formula, students actually already know the formula that will be used for solve the problem, but when writing the angle is wrong especially in the formula sine rules, whereas in the application of the cosine rule formula students still appear like memorizing so the formula isn't right. The next error is encoding error. The reason is because of the error in the previous stage so that it becomes the wrong answer. This error arises when students with medium initial abilities is writing the final answer, especially in the cosine rule, the answer is often students are still squared and have not been transformed into root shapes. Students with low initial ability levels tend to make transformation type mistakes error. The reason is the lack of mastery of the concept of sine and cosine rules, students just memorizing the formula without knowing how to apply it. The next is the process skills error. The reason is the lack of thoroughness of students when working and as a result of errors in the previous stage. Error the next is encoding error. The reason is students who are not confident with the answer and reluctant to check again.

Keywords: sine and cosine rules, newman’s error analysis, initial ability