Answer: d. 6.
Step-by-step explanation:
AD=3 DC=27 BD=x+3 x>0 x=?
1. Consider triangle ABD:
It's rectangular (∡ADB=90°). ⇒
AB²=(x+3)²+3²=(x+3)²+9.
2. Consider triangle BDC:
It's rectangular (∡BDC=90°). ⇒
BC²=(x+3)²+27²=(x+3)²+729.
3. Consider triangle ABC:
It's rectangular (∡ABC=90°).
AC=AD+CD=3+27=30. ⇒
AC²=AB²+BC²
(x+3)²+9+(x+3)²+729=30²
2*(x+3)²+738=900
2*(x²+6x+9)+738=900
2x²+12x+18+738=900
2x²+12x-144=0 |:2
x²+6x-72=0
D=324 √D=18
x₁=-12 ∉ 'cause x>0
x₂=6.
