Hi, and welcome to the forum.
It's not clear what you're trying to accomplish. You want to start a new line within the braces? And you really want two "+" signs in sequence, one at the end of the upper expression and another at the beginning of the lower expression? That's rather unusual, but if so:
n = {Y_пор} over {Y} ~approx~ {i overline b C_{y~макс}} over {13} ~ (1+e) over {e}
left lbrace stack { left[ {{4 -left (1 - {b_R} over {b_o} right ) [3 - {overline r}_o (1+ {overline r}_0)]}
over {12 (1- {{overline r}_o}^2)}} right] ~ Z^2 "+"
# "+" left[ {1 - {{1} over {2}} left (1 - {b_R} over {b_o} right )} over {(1 - {{overline r}_o}^2)} right ] ~
left ( {Ak^2-2} over {Ak^2-1} right )^2 k^2} right rbrace
If not, explain in detail why that is not what you want and how it should be changed.
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Mr. Programmer
AOO 4.1.7 Build 9800, MacOS 13.7.6, iMac Intel. The locale for any menus or Calc formulas in my posts is English (USA).