AE=AB-DC=60,DE=BC=80
According to pythagorean theorem AD= 100
2.sinA=DE/AD=4/5,cosA=AE/AD=3/5
PA=x,PM=PAsinA=4/5x,AM=PAcosA=3/5x
PN=AB-AM= 100-3/5x
S = PM× PN =- 12/25x 2+80x, X = PM/Sina ≥ 36 ÷ (4/5) = 45, that is, 45≤x≤ 100.
So s = pm× pn =- 12/25x 2+80x, 45 ≤ x ≤ 100.
3.- 12/25x^2+80x=3300,
12x^2-2000x+82500=0,
x 1=9 1.7,x2=75