Question

tìm số nguyên x thỏa mãn : -210=(-1)+(-2)+(-3)+....+(-x+1)+(-x)

Answer 1

-210 = (-1) + (-2) + (-3) + ... + (-x + 1) + (-x)

=> -210 = -(1 + 2 + 3 + ...+ x - 1 + x)

=> 210 = 1 + 2 + 3 + x - 1 + x

=> 210 = \(\frac{\left(x+1\right).x}{2}\)

=> 420 = (x + 1).x

=> 21 . 20 = (x + 1) . x

=> (20 + 1) . 20 = (x + 1) . x

=> x = 20

Answer 2

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Answer 3

x = 20 mình chắc chắn luôn 

bạn gặp trong vòng 13 cua thi violympic đúng ko ?

nếu đúng mong bạn k cho^_^