The study used a Pretest-Posttest Quasi Experimental Design to determine the effectiveness of the alternative general formula in finding the excess area of a regular polygon for the Grade-12 Senior High School STEM students of Tigao National High School which tapped 40 students as subjects of the study. The result showed that the subjects' mean scores increased greatly from the pretest to the posttest. The results indicated that using the alternative general formula increases students' achievement. As a result, using the alternative method was more beneficial and effective for students because it was easier for them to find the excess area of a regular polygon inscribed in a circle compared to using the original formula. The calculated P-value in pre-test is 0.102, which is greater than the alpha level of 0.05. It failed to reject the null hypothesis which means that there is no significant difference in the pretest mean scores. The computed P-value in posttest 0.000 is less than the 5% alpha level. It rejects the null hypothesis which implies that there is a highly significant difference between the original formula and the alternative general formula. The computed P-value of 0.615 in the interaction effect between the Formula Used and Mathematical ability is greater than the 5% alpha level. As a result, it may be inferred that the Formula used and the mental capacity have no meaningful interaction. The alternate formula, regardless of the students' mental capacity, provides a more efficient and helpful method for both groups, whether they belonged to the average or advanced group.