rewrite the trig expression. assume u>0 and the value of the inner inverse trig expression; 0<theta<π/2

I always like to draw a triangle to answer these.

We want to find the cosine of a first quadrant angle whose tangent = u.....  (or, u/1)

So we have....

cosΘ = 1/ √(1 + u²)

Nice triangle Melody! I like your answer.

Can somebody explain me this problem?

No you will have to wait a few more years!  I think that you already knew that.

Will you please stop your excessive posting!