Question

A1+a2+a3=180 solve for a1

Answer 1

A1 = 180 - (a2 + a3) ;
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or: 
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A1 = 180 - a2 - a3.
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Since there are no values given, we solve for "A1" by isolating "A1" and solving in terms of the remaining values.

Answer 2

Answer: a1=180-a2-a3....