Question

 tính nhanh 

A = 1x3 + 3x5 + 5x7 + .............+ 37x39 + 39x41

Answer 1

Ta có : A = 1.3 + 3.5 + 5.7 + ....... + 37.39 + 39.41

=> 6A = 1.3.5 - 1.3.5 + 3.5.7 - 3.5.7 + ....... + 39.41.43

=> 6A = 39.41.43

=> A = 39.41.43 / 6

=> A = 11459,5

Answer 2

A=1x3+3x5+5x7+...+37x39+39x41

6A=1x3x5-1x3x5+3x5x7-3x5x7+...+39x41x43

6A=39x41x43

A=\(\frac{39x41x43}{6}\)