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Me ! write an equation for the interior angles of this triangle that uses the triangle sum theorem

One expression below in undefined, and the other expression has a well defined value. which expression is undefined and explain why it has no value. which expression is defined? what is the value of that expression and how do you know? what is a different expression using a trig inverse function that is also undefined?

Fast! find the length of cu. the triangles are similar. show your work.