Hmm, I was little unclear with notation. After first line i just stopped to care about original values of a, b, c - they were used only to compute k and forgotten (with k = a * b / c function k * 1.05^(-x) is same as (a*b)/c * 1.05^(-x) )
And rest was just integration of function k * 1.05^(-x) on interval [a, b], whatever values of k, a, b are. And for integral of 5 * 1.05^(-x) on interval [1, 5] wolfram alpha gives same result (17.3042) - just fill there 5 * 1.05^(-x), from 1, to 5.
And rest was just integration of function k * 1.05^(-x) on interval [a, b], whatever values of k, a, b are. And for integral of 5 * 1.05^(-x) on interval [1, 5] wolfram alpha gives same result (17.3042) - just fill there 5 * 1.05^(-x), from 1, to 5.