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Bangun RTUang Kotak Tertutup
alas persegi panjang
p = 3l
V= p l t = 36
3l. l. t = 36
l² .t = 36/3
t = 12/l²

Luas  permukaaan tertutup
L = 2 (luas alas ) + keliling alas x tinggi
L = 2(pl ) + 2(p+l)t
L = 2(3l²) + 2(3l + l)(12/l²)
L = 6 l² + 8(12)/l
L = 6l² +  96l⁻¹
L' =0
12 l - 96 l⁻² =0
12 l - 96/l² = 0
12 l³ - 96 = 0
12 l³ = 96
l³ = 8
l = 2 dm
p = 3 l = 3(2)
p = 6 dm
l =