Геометрия. Правильная призма

SBCF1E1 - запись категорически неверная.
S (BCF1E1) = BC * BF1 = 5 * BF1
BF1 = V(BF^2 + FF1^2) = V(BF^2 + 10^2)
< A = (n-2)/n * 180 = (6-2)/6 * 180 = 120 град.
Треугольник BAF:
AB = AF и < A = 120 град. =>
< ABF = < AFB = (180-120)/2 = 30 град.
AK _|_ BF =>
AK = AB * cos ABF = AB * cos 30 = 5 * V3/2 = 5V3/2 =>
BF = 2*AK = 2 * 5V3/2 = 5V3
=>
BF1 = V(BF^2 + 10^2) = V((5V3)^2 + 10^2) = V175 = 5V7
S (BCF1E1) = BC * BF1 = 5 * BF1 = 5 * 5V7 = 25V7