將一個未知數開方再二次方...........

2次方和開方根抵消√x²
= x
但(√x)²即是將x開方再2次方..即是x
所以我想應該也是x吧

e.g:√5 =2.236067977
再2次方..不也是5嗎?!

No, take x = -1, you get
√-1 = i or -i, however (√x )² = -1
which is not equal to | -1| = 1

Note that in general √(x²) = | x | is true for all real x, but (√x )² = x is true for all positive real x, since any positive number under square rot sign must be positive (no absolute sign is needed.

(√x )² = x
我們可以將左右兩邊一同開方, 左右均得到 (√x ). 由此可證.

答案是X
因爲(√x )² 中已經保證x大於或等於0，二次方後可直接等於x,

注意，這是從實數角度考慮。虛數角度不用了，別鑽牛角尖。

未必會
在真數列 (real number), 這個是真的, 例如4, 開方後再sqrt之後亦是等於4. 即是等於正4.
不過在虛數列中 (complex number), 如果 -9, 開方後變成 3i, i^2 = -1, 當你sqrt. 左就會變成 3^2x(-1) = -9
所以(rt. x) ^2 之後的答案, 只可說成 x, 而不可以 |x|.