Fast martin simplified the composed function f(x)=sin(arctan x). his work is shown below. step 1: f(x)=sina, where a=arctan x and x = tana step 2: tana= opp/adj=x/1 step 3: hypotenuse= square root of 1^2-x^2= square root of 1-x^2 step 4: sina=opp/hyp=x/squareroot 1-x^2 in which step did martin make is first error?

We have that

Step 1: f(x)=sinA, where A=arctan x and x = tanA> is correct
Step 2: tanA= opp/adj=x/1> is correct
Step 3: hypotenuse= square root of 1²-x²= square root of 1-x²> is not correct
Because hypotenuse= square root of (1² + x²)=square root of (1+x²)
Step 4: sinA=opp/hyp=x/square root of (1+x²)

the answer is
In step 3 Martin make is first error