Why can't x^2+9 be factored? And is there a way to tell if a problem can be factored or not just by looking at it?

Write the expression as a constant , a single trigonometric function, or a power of a trigonometric function. sin x cos x/ tan x

Ascientist wants to use a model to present the results of his detailed scientific investigation. why would a model be useful? a )because the model makes the concepts easier to understand b) because the model is easy to put together and to use c) because the model prevents other scientists from asking questions d) because the model requires the audience to pay full attention to it i don't know what to choose because between a and d a is much more likely than d because it only wants to get the attention of the audience. i'm confuse.

An object in geometry with no width, length or height is a(n):