Tìm x, biết: ( 1 + x ) + ( 2 + x ) + ( 3 + x) + ….. + ( 200 + x ) = 21100

(1+x) +(2+x)+...+(200+x)=21100 <=> 1+x +2+x +3+x +...+200+x =21100 <=>1+2+3+...+200 +200x = 21100 Ta có 1+2+3+.. ..+200 =(1+200)+(2+109)+(3+108)+... =100×201 = 20100 (vì có ([1+200]-1) :2 =100 cặp ) =>200x = 21100-20100 =1000 <=> x=1000:200 =5