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
đúng nha
A = 1.2 + 2.3 + 3.4 + ... + 98.99
= 1.(1 + 1) + 2.(2 + 1) + 3.(3 + 1) + ... + 98.(98 + 1)
= 12 + 1 + 22 + 2 + 32 + 3 + ... + 982 +98
= (12 + 22 + 32 + ... + 982) + (1 + 2 + 3 + ... + 98)
= B + (1 + 98).98 : 2
= B + 4851
Do đó A = B + 4851 suy ra A - B = 4851
Kết luận : A - B = 4851
A - B = 1+2+3+4+...+99
=99.(99+1)/2
=99.100/2
=99.50
=4950
những câu này chỉ là trẻ con thôi
Dep trai loi tai ai bo dam vao mom cai
A= 1.2+2.3+3.4+...+98.99 3A= 1.2.3+2.3.3+3.4.3+...+98.99.3 3A= 1.2.3+2.3.(4-1)+3.4.(5-2)+...+98.99.(100-97) 3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+98.99.100-97.98.99 3A= 98.99.100 A= (98.99.100):3 A=323400
