(1)a4=82-1=7×9
a5=102-1=9×11
(2)an=(2n)2-1=(2n-1)×(2n+1)
(3)原式=1/1×3+1/3×5+…+1/4023×4025
=1/2(1-1/3+1/3-1/5+…+1/4023-1/4025)
=1/2(1-1/4025)
=1/2*4024/4025
=2012/4025
(1)a4=82-1=7×9
a5=102-1=9×11
(2)an=(2n)2-1=(2n-1)×(2n+1)
(3)原式=1/1×3+1/3×5+…+1/4023×4025
=1/2(1-1/3+1/3-1/5+…+1/4023-1/4025)
=1/2(1-1/4025)
=1/2*4024/4025
=2012/4025