11²-12²+13²-14²+15²-16²+17²-18²+19²-20
=(11+12) x (11-12) + (13+14) (13-14) .+ (19+20)(19-20)
= - 23 - 27 - .- 39共5项
= - (23+39) x 5 ÷2
=-155