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sn =3n square+4n
s1=3×(1)square+4×1=3+4=7
s2=3×(2)square+4×2=12+8=20
or, a=7,
now,a2=s2_s1=20_7=13
now,d=a2_a1=13_7= 6
since,an=a+(n_1)d
a25=7+(25_1)6
a25=7+24×6
a25=7+144
a25=151