Determine k and solve the equation x^3 -kx^2 + 20x +12 = 0.
One of its zeros, say , is triple of another, say . Thus, .
Thus, we have that,
Now we solve for ...
But, I have an easier way. Solving for that equation can get messy. Instead, let us hope that its solutions are integers. Not necessarily this approach might work but let us try it, if it works then we can save a lot of work. If it does not we can try another way.
Now, so . It must be that . Are factors of . Since must be a square and positive. It means . Thus, the solutions are . Now take the square root to remove the square on to get: .
Now which of these pairs satisfies equation 2, i.e. .
We easily see that is the only solution.