Question

a  = 1.2 +2.3+3.4+..... +19.20

tính  A.3

Answer 1

ta có :

A.3=1.2.3+2.3.3+3.4.3+.....+19.20.3

A*3=1.2.3+2.3.(4-1)+3.4.(5-2)+...+19.20. (21-18)

A*3=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+19.20.21-18.19.20

A.3=19.20.21

A.3=7980

Mình làm đúng rồi đó k đúng cho mình nha

Answer 2

a=1.2+2.3+3.4+...+19.20

a=2+6+12+20+30+42+56+72+90+110+132+156+182+210+240+272+306+342+380

a=2660

=>3.a=3.2660=7980

Đáp số: 7980

Answer 3

A = 1.2 + 2.3 + 3.4 + ... + 19.20

3A = 1.2.(3 - 0) + 2.3.(4 - 1) + 3.4.(5 - 2) + ... + 19.20.(21 - 18)

3A = 1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 19.20.21 - 18.19.20

3A = (1.2.3 + 2.3.4 + 3.4.5 + ... + 19.20.21) - (0.1.2 + 1.2.3 + 2.3.4 + ... + 18.19.20)

3A = 19.20.21 (bn tự tính nhé!)