2 + 4 + 6 + 8 + ... + 2n = 210
=> 2 . (1 + 2 + 3 + 4 + ... + n) = 210
=> 1 + 2 + 3 + 4 + ... + n = 210 : 2
=> 1 + 2 + 3 + 4 + ... + n = 105
=> n . (n + 1) : 2 = 105
=> n . (n + 1) = 105 . 2
=> n . (n + 1) = 210
Vì 14 . 15 = 210 => n = 14
b) 1 + 3 + 5 + ... + (2n - 1) = 225
<=> {[(2n - 1) + 1] . [(2n - 1) - 1] : 2 + 1} : 2 = 225
<=> (2n . 2n) : 4 = 225
<=> n2 = 225
=> n = 15.
2 + 4 + 6 +.......... + 2n = 210
1 * 2 + 2 * 2 + 2 * 3 + ..... + 2n = 210
2 ( 1 + 2 + 3 +...... + n ) = 210
1 + 2 + 3 +....... + n = 105
\(\frac{n\left(n+1\right)}{2}=105\)
n(n + 1 ) = 210
n(n+ 1 ) = 14 * 15
n( n + 1 ) = 14 ( 14 + 1 )
=> n = 14
Câu b làm tương tự
b) 1 + 3 + 5 + ... + (2n - 1) = 225
<=> {[(2n - 1) + 1] . [(2n - 1) - 1] : 2 + 1} : 2 = 225
<=> (2n . 2n) : 4 = 225
<=> n2 = 225
=> n = 15.