[tex]\bf 2[f(x)+g(x)]\implies 2f(x)+2g(x)\implies 2\cfrac{df}{dx}+2\cfrac{dg}{dx}\\\\
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\cfrac{1}{2}\left[ f(x)\cdot g(x) \right]\implies \cfrac{1}{2}\left[ \stackrel{product~rule}{\cfrac{df}{dx}\cdot g(x)+f(x)\cdot \cfrac{dg}{dx}} \right]
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\cfrac{1}{2}\cdot \cfrac{df}{dx}\cdot g(x)+\cfrac{1}{2}\cdot \cfrac{dg}{dx}f(x)[/tex]