Question

What is the equation of the tangent line of `f(x)=sin ^2x + cos^2 x` at `x=0`?

Answer 1

`y=1`.

Explanation:

Observe that `sin^2(x)+cos^2(x)=1`, so this function is actually a constant.
This means that its graph is the horizontal line `y=1`, and of course the tangent to a line is the line itself.
So, in every point (thus including `x=0`), the tangent to `y=1` is `y=1` itself.