设数列{a n }满足a 1 =2,a 2 +a 4 =8,且对任意n∈N * ,函数f(x)=(a n -a n+1 +a n+2 )x+a n+1 cos x-a n+2 sin x满足f′ =0. (1)求数列{a n }的通项公式; (2)若b n =2(a n + ),求数列{b n }的前n项和S n .
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