in diagram, what is AC?

Answer:
AC = 10 units
Step-by-step explanation:
In the given right angle triangle BCD
[tex]BC^{2}=BD^{2}+CD^{2}[/tex]
By putting BC = 17 and CD = 8
[tex]17^{2}= BD^{2}+8^{2}[/tex]
[tex]BD^{2}=17^{2}-8^{2}=289-64[/tex]
[tex]BD^{2}=225[/tex]
[tex]BD=\sqrt{225}=15[/tex]
Now in ΔACD
[tex]AC^{2}=AD^{2}+CD^{2}[/tex]
[tex]AC^{2}=(AB-DB)^{2}+CD^{2}[/tex] [since AD = AB-DB]
[tex]=(21-15)^{2}+8^{2}[/tex]
[tex]=6^{2}+8^{2}[/tex]
= 36 + 64
= 100
[tex]AC=\sqrt{100}=10units[/tex]