2. Perhatikan gambar disamping diketahui panjang EC = 30 cm Tentukan : a.panjang rusuk kubus B.luas permukaan kubus

misalkam panjang rusuk r
[tex] {ac}^{2} = \sqrt{{r}^{2} + {r}^{2}} \\ \: \: \: \: \: \: \: = \sqrt{ {2r}^{2}} \\ \: \: \: \: \: \: \: = r \sqrt{2} [/tex]
[tex] {ec}^{2} = {ae}^{2} + {ac}^{2} \\ \: \: \: \: \: \: \: = {r}^{2} + {(r \sqrt{2}) }^{2} \\ {30}^{2} = {r}^{2} + 2 {r}^{2} \\ 900 = 3 {r}^{2} \\ 300 = {r}^{2} \\ r = \sqrt{300} = 10 \sqrt{3} [/tex]
jadi panjang rusuknya
[tex]10 \sqrt{3} cm[/tex]

luas permukaan kubus
[tex] {6r}^{2} = 6 \times {(10 \sqrt{3}) }^{2} \\ \: \: \: \: \: \: \: = 600 \: {cm}^{2} [/tex]