[ (2²+4²+..+100²)-(1²+3²+...+99²) ] /(1+2+...+10+9+...+1 )
=[ 2(2²+4²+...+100²)-(1²+2²+...+100²) ] /100
=[ 8(1²+2²+...+50²) -(1²+2²+...+100²) ] /100
= [ 8(1/6)(50)(51)(101) - (1/6)(100)(101)(201) ] /100
= 4(1/6)(51)(101) - (1/6)(101)(201)
= 101/2