# Missing coordinates?

Can someone just explain this to me; my teacher did a poor job doing so as usual...

I'm trying to find the missing coordinate of P, using the fact that P lies on the unit circle in the given quadrant.

The question looks like this: P( , -7/25), quadrant IV

Is there a proper way to find this using my TI-83? If so, how? Thanks.

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HINTS: You’re given that $P$ is the point $\left(x,-\frac7{25}\right)$ on the unit circle in the fourth quadrant. The fact that $P$ is on the unit circle tells you that $$x^2+\left(-\frac7{25}\right)^2=1\;;$$ why? And what are the values of $x$ making this true?
The fact that $P$ is in the fourth quadrant tells you the algebraic sign of $x$; is $x$ positive, or is it negative?
All points on the unit circle obey the equation $x^2 + y^2 = 1$. You know $y=-\frac{7}{25}$, and can thus solve for $x$.
In case you forgot, in general, the circle around $(x_c,y_c)$ with radius $r$ is described by $$(x - x_c)^2 + (y - y_c)^2 = r^2$$