Lời giải:
$A=1.2+2.3+3.4+...+8.9+9.10$
$3A=1.2(3-0)+2.3(4-1)+3.4(5-2)+....+8.9(10-7)+9.10(11-8)$
$=(1.2.3+2.3.4+3.4.5+...+8.9.10+9.10.11)-(0.1.2+1.2.3+2.3.4+...+8.9.10)$
$=9.10.11$
$\Rightarrow A=\frac{9.10.11}{3}=330$
Tính tổng A = 1.2 + 2.3 + 3.4 + 4.5 + 5.6 + 6.7 + 7.8 + 8.9 + 9.10 .
Bài toán 3 : Tính tổng A = 1.2 + 2.3 + 3.4 + 4.5 + 5.6 + 6.7 + 7.8 + 8.9 + 9.10
A=1.2+2.3+3.4+4.5+5.6+6.7+7.8+8.9+9.10+...+2016.2017
1.1+1.2+2.3+3.4+4.5+5.6+6.7+7.8+8.9+9.10+10
1.2+2.3+3.4+4.5+5.6+6.7+7.8+8.9+9.10
A= 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/ 7.8 + 1/ 8.9 + 1/ 9.10
Tìm A:
A = 1.2+2.3+3.4+4.5+5.6+6.7+7.8+8.9+9.10
"." là dấu nhân nha!!!!
2/1.2+2/2.3+2/3.4+2/4.5+2/5.6+2/6.7+2/7.8+2/8.9+2/9.10 =?
tính tổng:
a,A=1.2+2.3+3.4+4.5+5.6+6.7+7.8+8.9
b,B=9.10+10.11+11.12+...+98.99
cảm ơn đã giải nha