# Math Help - differentiable/continuous functions

1. ## differentiable/continuous functions

Determine all real values of a and b such that the function
f(x) = ax+b for x =< 1
= tan(Pi*x/4) for 1 < x < 2
is differentiable at x = 1

?

2. Is this the correct function?

$f(x) = \begin{cases} ax+b, & \mbox{if } x \le 1 \\ \tan ( \pi x/4 ), & \mbox{if } 1

You basically need to check for two things:

1) The function must be equal on both sides as $x \to 1$
2) The function's derivative must be equal on both sides as $x \to 1$

For the first condition, plug in $x=1$ into both parts to get:

$a+b = \tan(\pi / 4)$

And the second condition, take derivatives of both sides, then plug in $x=1$ to get:

$a = \frac{\pi}{4} \sec^2 (\pi / 4)$

Once you have established these two equalities, you can now determine what $a,b$ are.