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