当n=1时,
a(1)=S(1)
=0.75×(1+1)
=1.5;
当n≥2时,
a(n)=S(n)-S(n-1)
=0.75(n+1)-0.75(n-1+1)
=0.75.
所以
a(n)=1.5(当n=1)
a(n)=0.75(当n≥2)