No 19 ya kak Dan caranyaaaaa

Dimensi Tiga

DB = diagonal =  2√2 cm
BG = diagonal = √(2²+x²)  = √(4+x²)
DG = diagonal = √(2²+x²) = √(4+x²)
sudut (DB, BG) = θ

Δ BDG , aturan cosinus
cos θ = (DB² + BG² - DG²)/ (2(DB)(BG)
cos θ = (8 + 4+x² - 4 - x²) / (2 (2√2)(√(4+x²)
cos θ=  8/ (4√ 2(4+x²)) =  2 /√ (8+2x²)

θ = 60°  --> cos 60 = 2/ √(8+ 2x²)
1/2 = 2/ √(8+ 2x²)
√(8 + 2x²) = 2x 2 = 4
8 + 2x² = 4²
2x² = 16- 8 = 8
x² = 4
x = 2 atau x = - 2 (TM)
x = 2