Question

tổng của tất cả các số hạng của dãy số : 1.1 + 1.3 + 1.5 + ... + 2.5 + 2.7 + 2.9 = ...

Answer 1

=1.(1+3+5+7+9) + 2.(1+3+5+7+9)

=(1+3+5+7+9).(1+2)

=(1+3+5+7+9).3

= 25.3 = 75 

Answer 2

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