First, let: `u = x^2+ 3` and so `y = sin u`.
cos x2 + 3
does not equal
cos(x2 + 3).
The brackets make a big difference. Many students have trouble with this.
Here are the graphs of y = cos x2 + 3 (in green) and y = cos(x2 + 3) (shown in blue).
The first one, y = cos x2 + 3, or y = (cos x2) + 3, means take the curve y = cos x2 and move it up by `3` units.
The second one, y = cos(x2 + 3), means find the value (x2 + 3) first, then find the cosine of the result.
They are quite different!