GENERAL FORM:
a2 = b2 + c2 - 2bc CosA
** Use this formula when you do not have a complete set of ratios ( a length of a side with its corresponding angle ).
Only use this formula (it's confusing & long) ONCE, and then use the sine law or any other formula to help you complete the trig question you have.
Use the cosine formula given above to substitue in the values:
a2 = b2 ="b + c2 - 2bc CosA
a2 = (7.8)2 + (9.0)2 - 2(7.8)(9.0)Cos112
a2 = 60.84 + 81 - (-52.5947)
a2 = 194.44
a2 = √194.44
a = 13.9
These sites are helpful for extra practice at using cosine law (step-by-step lessons & answers):
http://www.analyzemath.com/Geometry/cosine_law_problems.html
http://www.themathpage.com/aTrig/law-of-cosines.htm
http://dougsimmsonline.googlepages.com/cosinelaw <-- has a vid too!
Labels: cosine law, Jenny, sribe
