11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19
= ( 11 + 19 ) + ( 12 + 18 ) + ( 13 + 17 ) + ( 14 + 16 ) + 15
= 20 + 20 + 20 + 20 + 15
= 80 + 15
= 95
\(11+12+13+14+15+16+17+18+19\)
\(=\left(11+19\right)+\left(12+18\right)+\left(13+17\right)+\left(14+16\right)+15\)
\(=0+0+0+0+15\)
\(=15\)
Chúc bạn học tốt
Tính nhanh :
\(11+12+13+14+15+16+17+18+19\)
\(=\left(19+11\right)+\left(18+12\right)+\left(17+13\right)+\left(16+14\right)+15\)
\(=30+30+30+30+15\)
\(=30\times4+15\)
\(=120+15\)
\(=135\)