6*25^x = 9^x + 15^x6*5^(2x) = 3^(2x) + 3^x*5^x6*5^x/3^x = 3^x/5^x + 1,上下除以3^x*5^x,∵15^x≠06*(5/3)^x = (3/5)^x + 16 = (3/5)^x/(5/3)^x + (3/5)^x(3/5)^(2x) + (3/5)^x - 6 = 0令u=(3/5)^xu² + u - ...