Not the product rule - your teacher used the trigonometric identity .
First off i have a problem that the teacher somewhat worked out on the board.
I am not sure how he got the answer.
Find the vector function that represents the curve of intersection of the two surfaces.
and the surface
Now, from here i think the teacher used the product rule? I am not sure.
Solving for z
I tried the product rule but couldnt get it. Could someone work out the steps on how he got the answer?