15 58
Step-by-step explanation:
BD = DC = 1/2 BC ---> AD is median ( because in equilateral triangle altitude equals to median )
so, therefore
AB = 18 (given)
BD = 1/2 BC
BD = 1/2 × 18
BD = 9
AB² = AD² + BD² ---( by pythagoras theorm)
18² = AD² + 9²
324 = AD² + 81
324 - 81 = AD²
243 = AD²
then,
AD = √243
AD = 15 58
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