Ta có:

n.(n-1):2=10

=> n.(n-1)=20

=> n.(n-1)=5.4

=> n.(n-1)=5.(5-1)

=> n=5.

(x² - 1) (x² + 4x + 3) = 192

<=> (x + 1)(x - 1)(x + 3)(x + 1) = 192

<=>(x² + 2x − 3)(x² + 2x + 1) = 192 (*)

Đặt t = x² + 2x + 1

(*) => (t − 4)t = 192
<=> t² − 4t − 192 = 0
<=> (t + 12)(t − 16) = 0
<=> t + 12 = 0 hoặc t - 16 = 0

<=>t = -12 hoặc t = 16

=> x

19² à bạn

eaten, come, worked be, done, seen, been, haven, played, realized, taken, gone, known, lived, lost.