令x=1/4
2f(1/4)+(1/4)f(1/4)=5×(1/4)²
(9/4)f(1/4)=5/16
f(1/4)=20/144
2f(x)+(1/4)(20/144)=5x²
2f(x)=5x²-5/144
f(x)=5x²/2 -5/288