a(n+2)=(5/3)a(n+1)+(2/3)an ???????????
a(n+2) - 2a(n+1) = -(1/3)( a(n+1) - 2an)
{a(n+1) - 2an } 是等比数列, q= -1/3
a(n+1) - 2an = (-1/3)^(n-2). (a2 - 2a1)
=(-1/3)^(n-1)
a(n+1) -(7/9)(-1/3)^(n+1) = 2[an -(7/9)(-1/3)^n ]
{an -(7/9)(-1/3)^n} 是等比数列, q=2
an -(7/9)(-1/3)^n = 2^(n-1) . [a1 -(7/9)(-1/3)^1]
=(17/27).2^n
an =(7/9)(-1/3)^n +(17/27).2^n
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