# Thread: parametric Equations

1. ## parametric Equations

I am asked to sketch the graph of this equation using my TI-84 Plus calculator, changing from parametric to function mode to solve the equation X^2 + Y^2 = 4, for Y as a function of X. Given are y1 = +sqrt(?) and y2 = -sqrt(?)
I must be doing something wrong because when I solve the equation for y as a function of X, I come up with Y = Sqrt(4-x^2) and this gives me a syntax error on the calculator, so I know what I have done is wrong. Would someone please explain the error of my ways.
Thank You,
Keith Stevens

2. I don’t anything about the TI84. However, what you have given is not normally thought of as parametric equations. You have given just two rectangular equations for different semicircles.

The usual parametric equations for that circle are: x(t)=2cos(t) & y(t)=2sin(t).

3. Originally Posted by kcsteven
I am asked to sketch the graph of this equation using my TI-84 Plus calculator, changing from parametric to function mode to solve the equation X^2 + Y^2 = 4, for Y as a function of X. Given are y1 = +sqrt(?) and y2 = -sqrt(?)
I must be doing something wrong because when I solve the equation for y as a function of X, I come up with Y = Sqrt(4-x^2) and this gives me a syntax error on the calculator, so I know what I have done is wrong. Would someone please explain the error of my ways.
Thank You,
Keith Stevens
Hi, Keith,

you know that your calculator is able to solve the equation. The correct syntax is:

F2 + 1:
solve(x^2+y^2=4,y)
ENTER

You'll get the complete solution:
y=-√(4-x²) and x²-4≤0 or y=√(4-x²) and x²-4≤0

You find this line above the command line and you have to scroll to the right so that you can read the entire text.

The syntax error you got is only possible if you mixed up the operation sign of subtraction with the negative sign of numbers.

EB