[tex]\sum\limits_{i=3}^{10}(2i-9)=2(3)-9+2(4)-9+2(5)-9+2(6)-9+2(7)-9\\\\+2(8)-9+2(9)-9+2(10)-9\\\\=6-9+8-9+10-9+12-9+14-9+16-9+18-9+20-9\\\\=90-72=18[/tex]
