### Home > CALC > Chapter 4 > Lesson 4.4.2 > Problem4-142

4-142.

Find $a$ such that $f(x)$ is differentiable at $x = 1$

$f ( x ) = \left\{ \begin{array} { l l } { ( x + 2 ) ^ { 2 } - 3 } & { \text { for } x < 1 } \\ { a \operatorname { sin } ( x - 1 ) + 6 } & { \text { for } x \geq 1 } \end{array} \right.$

In order to prove that $f(x)$ is differentiable at $x = 1$, you must demonstrate that the derivatives agree from the left and right.

But do not forget that 'differentiablity implies continuity'. In other words, if the function is not continuous at $x = 1$, then it CANNOT be differentiable (even if the derivatives agree). So use the Three Conditions of Continuity to demonstrate that $f(x)$ is continuous at $x = 1$.

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Click the link at right for the full version of the eTool: Calc 4-142 HW eTool