If $n \geq 2$ is a positive integer, then the sum of the series ${ }^{n+1} \mathrm{C}_2+2\left({ }^2 \mathrm{C}_2+{ }^3 \mathrm{C}_2+{ }^4 \mathrm{C}_2+\ldots \ldots+{ }^n \mathrm{C}_2\right)$ is :
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