Tìm x biết rằng: x + (x+1) -(x+2) +...+(x+23)- (x +24)= 21

Đáp án: \(32\)

\(x + \left( {x + 1} \right) - \left( {x + 2} \right) + ... + \left( {x + 23} \right) - \left( {x + 24} \right) = 21.\)

\(x + \left[ {\left( {x + 1} \right) - \left( {x + 2} \right)} \right] + \left[ {\left( {x + 3} \right) - \left( {x + 4} \right)} \right] + ... + \left[ {\left( {x + 23} \right) - \left( {x + 24} \right)} \right] = 21.\)

\(x + \left[ {x + 1 - x - 2} \right] + \left[ {x + 3 - x - 4} \right] + ... + \left[ {x + 23 - x - 24} \right] = 21.\)

\(x - 1 - 1 - 1 - ... - 1 = 21\) (\(12\) số \(1\))

\(x - 12 = 21\)

\(x = 32.\)

Vậy \(x = 32.\)