# Mensuration problems

• February 1st 2008, 11:17 AM
BabyMilo
Mensuration problems
1a) Consider the triangle OAB
i) Find the area of the triangle
ii) Find the angle x
b) Find the area of the sector OAPB
c) Find the area of the shaded region.
d) Find the perimeter of the shaded region.
• February 1st 2008, 11:18 AM
BabyMilo
The diagram shows a pie slice. OAB is a triangle with a right angle at B. BCO is a sector of a circle whose centre is at O.

Calculate the area of the pie slice.
• February 1st 2008, 11:19 AM
BabyMilo
The diagram shows the construction of the Druid Temple stone circle in Scotland which is an example of an Egg-shaped circle of type 1.

Two identical right-angled triangles, ABC and ABD, are placed base to base. A semi-circle PXQ of radius r and centre A is drawn.
An arc with centre D is drawn from P until it meets the line DB at the point U.
An arc with centre C is drawn from Q until it meets the line CB at the point V.
An arc with centre B is drawn from U to V.

For the circle at Druid Temple:
AB= 3 megalithic yards; AC= 4 megalithic yards; r=7 megalithic yards.

Calculate the length of the perimeter of the stone circle.
• February 1st 2008, 06:33 PM
topsquark
Quote:

Originally Posted by BabyMilo
1a) Consider the triangle OAB
i) Find the area of the triangle
ii) Find the angle x
b) Find the area of the sector OAPB
c) Find the area of the shaded region.
d) Find the perimeter of the shaded region.

Use Heron's formula to find the area of the triangle. Then use the Law of Cosines to find angle x. Then use that angle x is a fraction of 360 degrees and hence your sector is the same proportion to the area of a circle.

c) and d) make no sense as there is no shaded region.

-Dan
• February 1st 2008, 06:38 PM
topsquark
Quote:

Originally Posted by BabyMilo
The diagram shows a pie slice. OAB is a triangle with a right angle at B. BCO is a sector of a circle whose centre is at O.

Calculate the area of the pie slice.

The area of the triangular portion is easy, just use the Pythagorean theorem to find the second leg of the triangle. Then if we calculate angle BOA (from some simple trig), which is supplementary to angle COB, that tells you what fraction of the circle the sector is.

-Dan
• May 14th 2008, 05:05 PM
yazeed
math failure!
Quote:

Originally Posted by BabyMilo
1a) Consider the triangle OAB
i) Find the area of the triangle
ii) Find the angle x
b) Find the area of the sector OAPB
c) Find the area of the shaded region.
d) Find the perimeter of the shaded region.

how the hel do u solve this?!
• May 14th 2008, 09:57 PM
earboth
Quote:

Originally Posted by BabyMilo
1a) Consider the triangle OAB
i) Find the area of the triangle
ii) Find the angle x
b) Find the area of the sector OAPB
c) Find the area of the shaded region.
d) Find the perimeter of the shaded region.

I've modified your sketch a little bit:

1. Split the triangle OAB into 2 right triangles.
2. Calculate the length of the red line ( $\sqrt{28}$)
3. Both right triangles form a rectangle whose area is:

$A_{OAB} = 6 \cdot \sqrt{28}=12 \cdot \sqrt{7}\approx31.75\ cm^2$

4. Use the Sine function to calculate the value of $\frac12 x$

5. The area of the sector is calculated by:

$\frac{A_{sector}}{\pi \cdot 8^2} = \frac{x}{360^\circ}$