訓練數學感 353 ─ 2 + 2 = 5

http://4rdp.blogspot.com/2023/06/353-2-2-5.html?m=0

$-20 = -20$

$16 - 36 = 25 - 45$

$16 - 36 + \frac{81}{4} = 25 - 45 + \frac{81}{4}$

$4^{2} - 2 \times 4 \times \frac{9}{2} + (\frac{9}{2})^{2} = 5^{2} - 2 \times 5 \times \frac{9}{2} + (\frac{9}{2})^{2}$

$(4 - \frac{9}{2})^{2} = (5 - \frac{9}{2})^{2}$

$4 - \frac{9}{2} = 5 - \frac{9}{2}$

$4 = 5$

$2 + 2 = 5$

請問上述計算步驟哪一步有錯？ 

 難度 ✩✩

常見的數學陷阱是除以零，不過這題是不同類型的錯誤。能設計出這題的人不簡單，數字配的剛剛好。