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Bearing and angle between planes

Two questions i have partly done.

1. Prove that in a plane triangle ABC

The bearing of an aeroplane from a fixed observation post is , the aeroplane being at a horizontal distance of 6 miles from the post. Calculate the flight direction of the aeroplane if, when it passes due east of the post, it is at a horizontal distance of 9 miles from the post. If the height of the aeroplane when its bearing is is *h* and its angle of elevation from the post is the same in both positions calculate, in terms of *h*, the height of the aeroplane when due east of the post.

In the figure

I first fin the length AB=

then angle PAX=[math\frac{9\sin 38^o}{5.65}=78.9^o[/tex]

Then using the opposite angles on parallel lines, the direction of the plane would be

Answer is supposed to be 131

2. A vertical mast stands on the north bank of a river with straight parallel banks running east-west. The angle of elevation of the top of the mast is when measured from A and point on the south bank *3a* to the east of the mast and when measured from another point B on the south bank distant *5a* to the west of the mast. Prove that the height of the mast is and that the angle of elevation measured from a point midway between A and B is given by the equation

I have proved .

But i don't know how to begin with the next part

Thanks