# Tangent Planes to Level Surfaces Examples 2

Recall from the Tangent Planes to Level Surfaces page that if we can represent a surface $S$ as a level surface of a three variable real-valued function $f(x, y, z) = k$, then the equation of the tangent plane at a point $P(x_0, y_0, z_0)$ on the surface can be calculated with the following formula:

(1)

Recall that this formula is especially useful when the variable $z$ is not isolated in the equation of the surface. We are now going to look at some examples of calculating tangent planes using the formula above.

## Example 1

**Find the equation of the tangent plane to the sphere $x^2 + y^2 + z^2 = 4$ at the point $(1, 1, \sqrt{2})$.**

Let $f(x, y, z) = x^2 + y^2 + z^2 = 4$. Then we have that the partial derivatives of $f$ are:

(2)

The partial derivatives evaluated at the point $(1, 1, \sqrt{2})$ is:

(3)

Therefore the equation of the tangent plane at $(1, 1, \sqrt{2})$ is given by:

(4)

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