Question

Tính nhanh

E = 1x1+2x2+3×3+...+15×15

Answer 1

E = 1 x 1 + 2 x2 + 3 x 3 + . . . + 15 x 15

E = 1( 2 -1 ) + 2 ( 3 -1 ) + 3 ( 4-1 ) + . . . + 15 ( 16 -1 )

E = 1.2 -1 + 2.3 - 2 + 3.4 - 3 + . . . + 15 . 16 - 15

E = ( 1.2 + 2.3 + 3.4+ . . . + 15 .16 ) - ( 1 + 2 + 3 + . . . + 15 )

E = 1360- 120

E  = 1240

Tk mk nha