suppose the three sides are a,b,c

The actual volume shall be abc

if each side is larger by 1 cm, the new volume shall be

(a+1)(b+1)(c+1)

=> (ab + a + b + 1) (c+1)

=> (abc +ab + ac +a + bc + b + c + 1)

=> abc + ab + ac + bc + a + b + c +1

let us calc the difference

=> a + b + c = 200 + 200 + 300 => 700

=> ab + bc + ca = 200*200 + 200*300 + 300*200 = 40 000 + 60 000 + 60 000

Total would be 160701

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