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by choosing Insert -> Other Macros -> Formatting -> Math Block (or search for Math Formula). Then enter the following in the Math Formula macro box:
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\left(\, \sum_{k=1}^n a_k b_k \right)^2 \le \left(\, \sum_{k=1}^n a_k^2 \right) \left(\, \sum_{k=1}^n b_k^2 \right)...
The following links might be helpful if you are new to LaTeX:...