Algebra
posted by Deanna .
I am using a TI83 calculator to find all the solutions to the quadratic equation (5x(Squared)  80 = 0)by graphing. The answer is supposed to be (4 and 4) but the calculator comes up with (ERR: NO SIGN CHNG). Can you tell me what I'm doing wrong?

Some numerical graphing/solving algorithms require the limits have different signs so that by marching through the interval, it is bound to find a root.
I do not know what Ti83 uses, and do not know which macro you used to graph the equation. Try graphing from 5 to 0 and from 0 to 5, each should come up with a root of x=4 and 4 respectively.
This problem applies to a numerical solution of general polynomial or other functions. If Ti83 has a special function for quadratic equations, it should not have this problem.