Why is sum of square root (given product) maximized with equal summands?

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Prove $\sum_{t=1}^{T}\sqrt{x_t-1}$, where $x_t\ge 1$ and $\prod_{t=1}^{T}x_t\le T$, gets its maximum when $x_t=T^{1/T}$.

Preliminary idea: The square root is a concave function. Sum of concave functions is concave.