# For what values of #x# does #f(x) = 4x + 8 sin x# have a horizontal tangent line?

##### 1 Answer

Aug 4, 2018

#### Explanation:

Recall that the **slope** of **tangent line (tgt.)** is given by

So, if the **tgt.** is **horizontal,** its **slope,** i.e.,

Now,

But,

Therefore,