Let straight f subscript straight n colon straight real numbers rightwards arrow straight real numbers, straight f subscript straight n open parentheses straight x close parentheses equals fraction numerator open parentheses negative 1 close parentheses to the power of straight n minus 1 end exponent over denominator open parentheses 2 straight n minus 1 close parentheses factorial end fraction straight x to the power of 2 straight n minus 1 end exponent and straight f open parentheses straight x close parentheses equals sum from straight n equals 1 to infinity of straight f subscript straight n open parentheses straight x close parentheses comma space straight x element of straight real numbers. Find straight f open parentheses straight pi over 3 close parentheses