5.12.21.34.53.80...即5,5+7,5+7+9,5+7+9+13,5+7+9+13+19,5+7+9+13+19+27,...,即5,2*5+2,3*5+2+2^2,4*5+2+2^2+2^3,5*5+2+2^2+2^3+2^4,6*5+2+2^+2^3+2^4+2^5,...,即该数列为{an},则通项an=5n+2+2^2+2^3+2^4+...+2^(n-...