Question

[nguyen phuong chi]

tim x

x : 3 + 1 - 2 + 3 - 4 + 5 - 6 + .......... + 97 - 98 + 99 = 127

Answer 1

[Trần Gia Đạo]

X = 3 x (1 + 3 + 5 +......+ 97 + 99) - (2 + 4 + 6 + ... + 96 + 98)

X = 3 x {(1 + 99) : 2  x [(99 - 1) : 2 + 1]} - {(2 +98) : 2 x [(98 - 2) : 2 + 1]}

X = 3 x 50 x 50 - 50 x 49

X = 3 x 50 x (50 - 49) = 150