# ISI2015-MMA-72

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The map $f(x) = a_0 \cos \mid x \mid +a_1 \sin \mid x \mid +a_2 \mid x \mid ^3$ is differentiable at $x=0$ if and only if

1. $a_1=0$ and $a_2=0$
2. $a_0=0$ and $a_1=0$
3. $a_1=0$
4. $a_0, a_1, a_2$ can take any real value
in Calculus
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