SOLUTION: find the measure of angle A in triangle ABC with vertices at A(7,4), B(1,7) and C(-3,-4)

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Question 1086197: find the measure of angle A in triangle ABC with vertices at A(7,4), B(1,7) and C(-3,-4)
Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
Use the dot product,
AB=(1-7,7-4)=(-6,3)
AC=(-3-7,-4-4)=(-10,-8)
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cos%28A%29=%28AB%2AAC%29%2F%28abs%28AB%29abs%28AC%29%29

cos%28A%29=%2836%29%2F%28sqrt%2845%29sqrt%28164%29%29
cos%28A%29=%286sqrt%28205%29%29%2F205
A=65.2