SINCE, I am assuming x is heading towards zero and we know that cosine of zero is 1.
Also I plan to use (sin x)/x goes to 1 as x goes to 0.
x^3 - 15x + 1 = 0 is a continuous function. So find some x's where the function is negative, positive...
By the intermediate value theorem (think, the price is right) the function must equal zero between those x values.