设f(x)=ax+b
f(2)=2a+bf(5)=5a+bf(4)=4a+b
[f(5)]^2=f(2)×f(4)
(5a+b)^2=(2a+b)(4a+b)
17a^2+4ab=0
又∵f(8)=8a+b=15
解方程得a=0b=15 或 a=4b=-17
若f(x)=15
f(2n)=15
f(2)+f(4)+……+f(2n)=15n
若f(x)=4x-17
f(2n)=8n-17
f(2)=8-17=-9
f(2)+f(4)+……+f(2n)=(-9+8n-17)×n/2=n(4n-13)