sn =(a 1+an)* n/2 =(a2+an- 1)* n/2 =(3+ 17)* n/2 = 100

n= 10

Using cosine theorem

cosADB=(AD^2+BD^2-AB^2)/2AD*BD

cosadc=-cosadb=(ad^2+cd^2-ac^2)/2ad*cd

CD=BD

BD= root 6