goi x (m) la chieu dai
ĐK: x>12 , x>0
khi do: chieu rog la: x-12 (m)
dien tich ban dau la: x(x-12) m2m2
chieu dai sau khi giam: x-1 (m)
chieu rog sau khi tang: x-12+2= x-10 (m)
dien tich sau khi doi CD,CR:(x-1)(x-10) m2m2
theo de ta co pt:
x(x-12)+30=(x-1)(x-10)
⇔x2−12x+30=x2−10x−x+10⇔x2−12x+30=x2−10x−x+10
⇔x=20(TMĐK)⇔x=20(TMĐK)
chieu rog la: 20-12=8 m
dien tich ban dau la: 20.8=160 m2m2
vay dien tich ban dau la 160m2