Question

[nguyễn hà trang]

• A=1.2+2.3+3.4+...+98.99
• B= 1²+2²+3²+...+98²
• ^{Tinh A-B}

Answer 1

[Thanh Tùng DZ]

sorry , ko để ý,

 Ta có A =1.2 + 2.3 + 3.4 + ...+ 98.99 
B = 1^2 + 2^2 + 3^2 +...+98^2 = 1.1+2.2+3.3+...+98.98 
Suy ra: A-B= (1.2 + 2.3 + 3.4 + ...+ 98.99) - (1.1+2.2+3.3+...+98.98) 
= (1.2-1.1) + (2.3-2.2) + (3.4-3.3) +...+ (98.99-98.98) 
= 1(2-1) + 2(3-2) + 3(4-3) +...+ 98(99-98) 
= 1.1 + 2.1 + 3.1 +...+ 98.1 
= 1+ 2+ 3+...+ 98 = [98.(98+1)]/2= 98.99/2 = 4851 

Answer 2

[Thanh Tùng DZ]

A = 1.2 + 2.3 + 3.4 + ... + 98.99

A x 3 =1.2.3 + 2.3.3 + 3.4.3 + ... + 98.99.3

A x 3 = 1.2.3 + 2.3.(4-1 ) + 3.4.(5-2 )+...+98.99.(100-97)

A x 3 = 1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+98.99.100-97.98.99

A x 3 = 98.99.100

=> A = 98.99.100:3

=> A = 323400