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Complete works of mathematics problems in grade three
1. The vertical foot crossing with direction D AB is E.

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