We will mark the key points as shown in the figure and draw the height BD.
The height of BD is also a median and a bisector (according to the third property of an isosceles triangle).
Triangle area ABC SABC = (1/2) AC * BD
Since BD is the median, AC = 2AD
SABC = (1/2) 2AD * BD = AD * BD
Since BD is also a bisector, ∠ABD = ∠ABC / 2 = 60 °
AD = AB * sin (∠ABD) = AB * sin60 °
BD = AB * cos (∠ABD) = AB * cos60 °
SABC = AB * sin60 ° * AB * cos60 ° = AB2 (√3 / 2) * (1/2) = AB2√3 / 4 = 1600√3
AB ^ 2/4 = 1600
AB ^ 2 = 1600 * 4
AB = 40 * 2 = 80
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.