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## Proofs with inscribed shapes

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# Proof: perpendicular radius bisects chord

CCSS.Math:

## Video transcript

- [Instructor] So we have this circle called circle O, based on the point at its center, and we have the segment OD, and we're told that segment OD, is a radius of circle O, fair enough. We're also told that segment OD is
perpendicular to this chord to chord AC, or two segment AC. And what we wanna prove is
that segment OD bisects AC. So another way to think about it, it intersects AC at AC's midpoint. So pause this video and see if
you can have a go with that. All right, now let's go
through this together. And the way that I'm going to do this, is by establishing two
congruent triangles. And let me draw these triangles. So I'm gonna draw one
radius going from O to C and another from A to O. Now we know that the
length AO is equal to OC because AO and OC, both radii. In a circle, the length of
the radius does not change. So I can put that right over there. And then we also know that OM is going to be congruent to itself, it's a side in both of these triangles. So let me just write it this way. OM is going to be congruent to OM and this is reflexivity. Reflexivity kind of obvious. It's going to be equal to itself, it's going to be congruent to itself. So you have it just like that. And now we have two right triangles. How do I know they're right triangles? Well, they told us that segment OD is
perpendicular to segment AC and our assumptions in our given. Now if you just had two triangles, that had two pairs of congruent sides that is not enough to establish
congruency of the triangles. But if you're dealing
with two right triangles, then it is enough. And there's two ways to think about it. We had thought about the RSH postulate where if you have a right triangle or two right triangles, you have a pair of sides are congruent, a pair and the hypothesis are congruent that means that the two
triangles are congruent. But another way to think about it, which is a little bit of common sense is using the Pythagorean theorem. If you know two sides of a right triangle, the Pythagorean theorem would tell us that you could determine
what the other side is. And so what we could say is and let's just use RSH for now, but you could also say we can
use the Pythagorean theorem to establish that AM is
going to be congruent to MC, but let me just write it this way. I will write that triangle, AMO is congruent to triangle CMO by RSH. And if the triangles are congruent then the corresponding
sides must be congruent. So, therefore, we know that AM, AM, segment AM is going to be, I'm having trouble writing congruent is going to be congruent to segment CM, that these are going to
have the same measure. And if they have the same measure, we have just shown that
M is the midpoint of AC or that OD bisects AC. So, let me just write it that way. Therefore, OD bisects AC. Segment OD bisects
segment AC and we're done.