Sd1 Sd2 cos(α), where Sd1 and Sd2 are the lengths of the diagonals and α is the angle between them.

Given that the length of the diagonal Sd1 is 26 cm and the angle between the diagonals is 30°, we can use the formula:

Srbmn = 2 26 26 cos(30°)
Srbmn = 2 676 0.866 (cosine of 30°)
Srbmn = 2 585.616
Srbmn = 1171.232 cm²

Therefore, the area of the rectangle RBMN is 1171.232 square centimeters.