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You can think of any triangle as half of a parallelogram! This is why its area is half that of the parallelogram: half of height times base. Watch this lecture series and complete the interactive exercises to practice using the triangle area formula.
Area of a triangle
Source: Khan Academy, https://www.khanacademy.org/math/cc-sixth-grade-math/cc-6th-geometry-topic This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License.