$
Integration by parts states that $\int u d v=u v-\int v d u,$ where $u$ and $v$ are differentiable functions.
In this case, we can choose $u=x$ and $d v=f^{\prime \prime}(x) d x.$
Then, we have $d u=d x$ and $v=\int f^{\prime \prime}(x) d x=f^{\prime}(x)+C,$
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