I don't see the reason to write this:

cout << fixed << setprecision(7) << calc(round(b * 1000000)/1000000) << endl;

This would lose accuracy of your answer, right?

The following is correct one.

cout << fixed << setprecision(7) << calc(b) << endl;

Another mistake is here:

while (c - b > 0.0000000001) // 1e-10
{
	/**************/
}

The error on number x will be increased by about n times when calculating weakness. So it should be 1e-12.

BTW, it is better to run a certain number of iteration for binary(ternary) search on float number.