Question

1 nhânv2 + 2 nhân 3+ 3 nhân 4 + ... + 49 nhân 50

Answer 1

đặt A = 1.2 + 2.3 + 3.4 + ... + 49.50

3A = 1.2.3 + 2.3.3 + 3.4.3 + ... + 49.50.3

3A = 1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + ... + 49.50.(51 - 48)

3A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 49.50.51 - 48.49.50

3A = 49.50.51

A = 49.50.51 : 3

A = 41650

Answer 2

A = 41650

Answer 3

 S = 1 . 2 + 2 . 3 + 3 . 4 + ... + 39 . 50

3S = 1 . 2 . 3 + 2 . 3 . ( 4 - 1 ) + 3 . 4 . ( 5 - 2 ) + ... + 49 . 50 . ( 51 - 48 )

3S = 1 . 2 . 3 + 2 . 3 . 4 - 1 . 2 . 3 + 3 . 4 . 5 - 2 . 3 . 4 + ... + 49 . 50 . 51 - 48 . 49 . 50

3S = 49 . 50 . 51

3S =124950

S = \(\frac{124950}{3}\)= 41650

Answer 4

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