a5²=a1×a13
a1²+8a1d+16d²=a1²+12a1d
a1=4d
a3=a1+2d=6d=3 d=0.5 a1=2
a20=a1+19d=2+19×0.5=11.5
S20=(a1+a20)×20÷2=135