bagaimana cara mengerjakannya ( pakai cara )

AB = lebar bidang diagonal = 8 cm
BG = panjang bidang diagonal
BG² = BC² + CG²
BG² = 12² + 9²
BG² = 144 + 81
BG² = 225
BG = √225
BG = 15 cm

luas bidang diagonal = p×l
luas bidang diagonal = BG × AB
luas bidang diagonal = 15 × 8
luas bidang diagonal = 120 cm²