(2/x+5)²-(2/x-5)²
= {（2/x)^2 + 2*(2/x)*5 + 5^2 } - {（2/x)^2 - 2*(2/x)*5 + 5^2 }
= 4*(2/x)*5
= 40/x
(2/x+5)²-(2/x-5)²
= { (2/x+5+(2/x-5)} { (2/x+5)-(2/x-5)}
= 2*(2/x) * 2*5
= 40/x