SOLUTION: Hello! I have a question concerning the following problem. I know your supposed to use the COS formula however I do not know how to approach that method. Could you show me step -by

Question 31180: Hello! I have a question concerning the following problem. I know your supposed to use the COS formula however I do not know how to approach that method. Could you show me step -by- step?
Find the area of a parallelogram whose sides are 6 inches and 8 inches and whose longer diagonal is 12 inches.
Thank you so much!
Answer by longjonsilver(2297) (Show Source): You can put this solution on YOUR website!
Given ANY triangle ABC where

length a is opposite angle A
length b is opposite angle B
length c is opposite angle C

then knowing just 3 sides, we can find an angle using the cosine rule, which states that:

so put the values in, re-arrange to find cosA and hence find the angle A.

Note: we could have the formula as
or

it is completely flexible.

jon.

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