They are essentially the same. Compound interest is the interest calculated at the rate of 100%. If the fund is A, the sum of principal and interest after one year is a+a, that is, a( 1+ 1). 2a means it is twice the capital. After two years, the sum of principal and interest is 2a× (1+1) = 4a = a (1+1)? ..... accordingly, the sum of principal and interest after n years is a (1+1) n. The quadratic function is the interest rate, the independent variable x, and the number of years is 2, so the sum of principal and interest is y = a (1+x%)?