# Proving two lines are perpendicular to each other

• Jun 26th 2012, 04:40 AM
etude
Proving two lines are perpendicular to each other
Hi all,

I have spent hours trying to solve this maths problem but unfortunately I just cannot figure out how this can be solved.

The question reads: The figure shows a square ABCD. M and N are points on AB and BC respectively such that BM=BN. Take a point P on MC such that BP is perpendicular to MC. Prove that PD is perpendicular to PN.

Attachment 24166
• Jun 26th 2012, 07:59 AM
mfb
Re: Proving two lines are perpendicular to each other
Are you allowed to use trigonometric relations?
The triangles BPM and CPB are similar, this allows to find coordinates of the point P. Together with D and N, you can calculate the angle.
• Jun 27th 2012, 04:31 AM
etude
Re: Proving two lines are perpendicular to each other
Can you please elaborate further? I fail to see why BPM and CPD are similar triangles, and even if this is the case I don't know how to obtain the coordinate of P.

And how do I work out the angle once I know the coordinate of P?

• Jun 27th 2012, 01:50 PM
mfb
Re: Proving two lines are perpendicular to each other
Both triangles have a 90°-angle and the angle BMC and MCB add up to 90°, therefore the angle BMC is equal to the angle CBP.

Quote:

And how do I work out the angle once I know the coordinate of P?
Scalar product of the corresponding vectors. But I think this answes my question as "no".
• Jun 28th 2012, 04:36 AM
BobP
Re: Proving two lines are perpendicular to each other
Start by showing that the triangles DCP and NBP are similar.

$\angle DCP=\angle CMB=\angle NBP,$
so

$\angle DCP=\angle NBP...............................(1)$

Let $\frac{AB}{MB}=\frac{CB}{NB}=k,$
then

$\frac{CD}{NB}=k................................... ..........(2)$

In the triangles $CBP$ and $CBM,$ we have

$\frac{CP}{BP}=\frac{CB}{BM}=k,$ (both ratios are the tangents of the equal angles $PBC$ and $PMB$).

$\frac{CP}{BP}=k................................... ..........(3)$

$(1),(2)$ and $(3)$ together show that the triangles DCP and NBP are similar.

From that it follows that the angles $CDP$ and $BNP$ are equal in which case angles $CDP$ and $CNP$ sum to $180$ degrees.

It then follows that since the interior angles of a quadrilateral sum to $360$ degrees, that, (from the quadrilateral $DCNP$) that the angles $DCN$ and $DPN$ sum to $180$, in which case $DPN=90$ degrees.
• Jun 28th 2012, 06:20 AM
etude
Re: Proving two lines are perpendicular to each other
Hi Bob,

I am sure what you posted is 100% correct.

I am almost there understanding what you are saying.

Can you please tell me why angle DCN + angle DPN = 180 degrees?
• Jun 28th 2012, 06:38 AM
BobP
Re: Proving two lines are perpendicular to each other
$\angle CNP= 180-\angle BNP=180-\angle CDP,$

$\therefore\qquad \angle CNP+\angle CDP=180.$

The sum of the interior angles of a quadrilateral is 360, so the sum of the remaining two must also be 180.