11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19
= (11 + 19) + (12 + 18) + (13 + 17) + (14 + 16) + 15
= 30 + 30 + 30 + 30 + 15
= 120 + 15
= 135
\(11+12+13+14+15+16+17+18+19\)
\(\Leftrightarrow\left(11+19\right)+\left(12+18\right)+\left(13+17\right)+\left(14+16\right)+15\)
\(\Leftrightarrow30+30+30+30+15\)
\(\Leftrightarrow\left(30.4\right)+15\)
\(\Leftrightarrow120+15=135\)
= (11 + 19 ) + ( 12 +18 ) ( 13 + 17 ) ( 14 + 16 ) + 15
= \(30+30+30+30+15\)
= \(30\times4+15\)
\(=120+15=135\)
Ta có:
(11+19)+(12+18)+(13+17)+(14+16)+15
=30+30+30+30+15
=60+60+15
=120+15
=135