2a=3b
a=3b/2
(a^2+b^2)/(a+b)^2
=[(3b/2)^2+b^2]/(3b/2+b)^2
=[9b^2/4+b^2]/(5b/2)^2
=[13b^2/42]/(25b^2/4)
=13/25
a^2+b^2/(a+b)^2
=(4a²+4b²)/(2a+2b)²
=(9b²+4b²)/(5b)²
=13/25
a=3/2b a^2 + b^2 = 9/4b^2 + b^2 =13/4 b^2
(a+b)^2 = (5/2 b)^2 = 25/4 b^2
原式 = 13/25