The sum of 2fifteens and 10 fifteens

In the proof of the triangle angle sum theorem, you must draw an auxiliary line. which construction must you use to draw this line?

Need ! give step by step solutions on how to solve number one \frac{9-2\sqrt{3} }{12+\sqrt{3} } number two x+4=\sqrt{13x-20} number three (domain and range) f(x)=2\sqrt[3]{x} +1

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?