Question

In the △ABC, the height AN = 24 in, BN = 18 in, AC = 40 in. Find AB and BC.

Answer 1

Answer:

AB = 30 inch

BC = 14 inch or BC = 50 inch

Step-by-step explanation:

Make a drawing. Please see the attachment. All measurements are in inches.

Consider the two triangles:

∆ ABC and ∆ AB'C

1. Looking only at ∆ ABC. Since this is not a rectangular triangle, let's first consider ∆ ANC, so we can calculate NC.

AN is the height and there fore is perpendicular to NC and thus also to BC.

In ∆ ANC

AN² + NC² = AC²

We want to calculate NC, so:

NC² = AC² - AN²

Given: AC = 40 and AN = 24

NC² = 40² + 24²

NC² = 1600 + 576

NC² = 1024

NC = +-SQRT(1024)

NC = 32

NC = NB + BC

We want to know BC

BC = NC - NB

Given: NB = BN = 18 and we just calculated NC to be 32 so...

BC = 32 - 18

BC = 14

2. Looking only at ∆ AB'C

AN is the height and there fore is perpendicular to B'C. Let's consider ∆ AB'N, so we can calculate AB'.

AN is the height and there fore is perpendicular to B'N, which means it has an angle of 90° in ∆ AB'N.

In ∆ AB'N

c² = a² + b²

AB'² = AN² + B'N²

Given: AN = 24 and B'N = 18

AB'² = 24² + 18²

AB'² = 576 + 324

AB'² = 900

AB' = +-SQRT(900)

AB' = 30

In ∆ AB'C

with AB' = 30 and AC = 40

c² = a² + b²

B'C² = AB'² + AC²

B'C² = 30² + 40²

B'C² = 900 + 1600

B'C² = 2500

B'C = +-SQRT(2500)

B'C = 50

Now we have our answers.

Extra:

Please look at the picture again, but now concentrate on the indicated 50 and - 50...

I am trying to explain something about the meaning of the outcome of mathematical calculations like:

c² = 2500

c = +- SQRT(2500)

c = 50 or c = - 50

Depending on where you want to start from, you can "move" 50 inch in one direction or 50 inch in the opposite direction, hence the -50 inch. Please let me explain why I am making a fuss...

1). Let's consider ∆ AB'C. Going from B' to C, you "move" -50 inch form B' towards C. This 50 inch is what we calculated earlier, but the we neglected to explain why we discarded the minus value of the SQRT... We just stated it to be only the positive value! Normally we give no meaning to the negative variant of it... Well, is this true in this case?

2). Let's consider ∆ ABC'. Going from B to C', you "move" 50 inch form B towards C'. Please understand that the direction is opposite that of - 50 inch in 1).

It is important to understand that a negative sign means the 180° in the direction of the other way.

In general. When calculating a square side by using the SQRT, you carefully need to consider if you can discard the -

minus value of your calculated outcome.

Not always, but usually there is some sort of meaning to the negative part of the SQRT, you just need to be willing to understand what it possibly could mean.

I hope this has made some sense to you :-).