ⅰ距A城X㎞,則距B城（100-X）㎞
∴X≥10,100-X≥10
10≤X≤90
ⅱ A城供電費用y1=〖λx〗^2 m_1
B城供電費用〖y2=λ(100-x)〗^2 m_2
則總費用〖y=λx〗^2 m_1+〖λ(100-x)〗^2 m_2
即y=〖2.5×20×x〗^2+〖2.5×10×(100-x)〗^2 x^2
即50x^2+〖25(100-x)〗^2≥2√(〖50x〗^2 25〖(100-x)〗^2 )
當且僅當√50 x=5(100-x)時,不等式左側取最小值.
∴5（√2+1）x=500∴x≈41.4㎞