a(7)=1
a(20)=25
a(25)=?
a(n)=a(1)+d(n-1)
a(7)=a(1) +6d=1 -11/13+6*24/13=
a(20)=a(1)+19d=-25
a(1) +6d=1
a(1)+19d=-25 (subtract first equation from the second)
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19d-6d =-26
13d =-26
d=-2
a(1)=1-6d=1-6*(-2)=13
a(25)= a(1)+d(25-1)= 13+(-2) *24= 13-48=-35
