Di jawab ya guys,pake cara pliss

L ABC = L ADB + ADC
1/2 x a x t = (1/2 x AB x AD x sin 45) + (1/2 x AC x AD x sin 45)
1/2 x 9 x 12 = (1/2 x 9 x AD x 1/2√2) + (1/2 x 12 x AD x 1/2√2)
54 = (9/4√2 x AD) + (3√2 x AD)
54 = AD x (9/4√2 + 3√2)
54 = AD (21/4√2)
AD = 216/21√2
AD = 36/7√2
AD = 7.2 atau 7 1/5 cm