3(a3+a5)=3(a1+a7)
3(a3+a5)+2(a7+a10+a13)=5a7+3a1+2a10+2a13=24
a1+a13=2a7, a1+2a10=3a7
4a7+5a7+3a7=24 a7=2
s13=(a1+a13)*13/2=13a7=26
3(a3+a5)=3(a1+a7)
3(a3+a5)+2(a7+a10+a13)=5a7+3a1+2a10+2a13=24
a1+a13=2a7, a1+2a10=3a7
4a7+5a7+3a7=24 a7=2
s13=(a1+a13)*13/2=13a7=26