3+8+15+24+35+48+...+2400
=1²-1+2²-1+3²-1+4²-1+5²-1+···+49²-1
=(1²+2²+3²+4²+5²+···+49²)-1×49
=49×(49+1)×(2×49+1)÷6-49
=40376