Posted by **Alexis** on Tuesday, November 3, 2009 at 6:17pm.

A billboard rests on top of a base. From a point level with the bottom of the base, the angle of elevation of the bottom of the billboard is 26 degrees and the top of the billboard is 31 degrees. From a point 21 meters further away, the angle of elevation to the top of the billboard is 17 degrees. Find the height of the billboard.

- Trig -
**Reiny**, Tuesday, November 3, 2009 at 6:51pm
Let's make a diagram.

Call the top of the billboard A and the bottom of the billboard E, and the point below AE we will call C, where C is at ground level.

Label D as the first point of observation, and B as the second point of observation, so that BD = 21

Let's look at triangle ABD.

Angle BDA = 180-31 = 149º

then angle BAD = 14º

and angle B = 17º

So by the sine law,

AD/sin17 = 21/sin14

We can find AD.

Now let's look at triangle ADC which is right-angled.

cos 31 = DC/AD, thus we can find DC

also sin 31 = AC/AD, giving us AC

and finally in triangle DEC

tan 26 = EC/DC giving us EC

so the billboard's height is AC - EC

Do not round off preliminary answers, good luck.

- Trig -
**drwls**, Tuesday, November 3, 2009 at 6:52pm
Let a be the original horizontal distance from which the billboard is viewed.

Let h be the height of the billboard.

Let b be the height of the base.

b/a = tan 26 = 0.4877

(b+h)/a = tan 31 = 0.6009

(b+h)/(a + 21) = tan 17 = 0.3057

You have to solve three equations in three unknowns (a, b, h) to get h.

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