Question

TÍNH NHANH : A = 1.3 + 3.5 + ... + 97.99

Answer 1

A = 1.3 + 3.5 + 5.7 + ...+ 97.99

6.A = 1.3.(5 + 1) + 3.5.(7 - 1) + 5.7.(9 - 3) + ...+ 97.99.(101 - 95)

6.A = 1.3.5 + 1.3 + 3.5.7 - 1.3.5 + 5.7.9 - 3.5.7 + ...+ 97.99.101 - 95.97.99

6.A = 1.3 +  (1.3.5 + 3.5.7 + 5.7.9 + ...+ 97.99.101) - ( 1.3.5 + 3.5.7 + ...+ 95.97.99)

6.A = 1.3 + 97.99.101 = ...=> A = 

Answer 2

A=1 x 99 x 36 cặp = 3564

Answer 3

Ta có

A = 1.3 + 3.5 + 5.7 + ...+ 97.99

6.A = 1.3.(5 + 1) + 3.5.(7 - 1) + 5.7.(9 - 3) + ...+ 97.99.(101 - 95)

6.A = 1.3.5 + 1.3 + 3.5.7 - 1.3.5 + 5.7.9 - 3.5.7 + ...+ 97.99.101 - 95.97.99

6.A = 1.3 +  (1.3.5 + 3.5.7 + 5.7.9 + ...+ 97.99.101) - ( 1.3.5 + 3.5.7 + ...+ 95.97.99)

làm tiếp nha bn 

hok tốt