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Unit 1, Lesson 2, Investigation 3: The Law of Cosines

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Ithaca High School Lesson Plans

Content Standards and Benchmarks:

II,3,1: Select and use appropriate tools; make accurate measurements using both metric and common units, and measure angles in degrees and radians.

          II,3,5: Use proportional reasoning and indirect measurements including applications of trigonometric ratios, to measure inaccessible distances and to determine derived measures such as density.



After the class has experimentally found the length of a third side of a triangle given two fixed sides and a varying interior angle, students will discover the Law of Cosines and know when it is applicable.


Anticipatory Set:

          Warm-up: “When do we use the Law of Sines?”

(Given a pair of opposites (side and angle) and a side or angle you can find the other opposite angle or side respectively)

        “What happens when you are given two sides and their included angle? Then what do you use? (segway)



Students will begin investigation 2 of lesson 2 today.  I will guide them to a deeper understanding of the Law of Cosines by giving multiple representations of when and how to apply it and by asking carefully designed questions.


Guided Practice:

Students will work through the next few problems in the text as I circulate and guide them through the problems. 


Independent practice:

Students will not have homework tonight!



Class discussion on when to use the Law of Cosines

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