The sides of a triangular field which contains 15 acres are in the continued proportion 3:5:7. Find the sides. (1 acre = 160 sq. rd.)
Draw triangle ABC with sides 3x,5x,7x using 7x as the base.
If A is the angle between the sides of 7x and 5x, then angle A can be found using the law of cosines.
3^2 = 5^2 + 7^2 - 2*5*7 cosA
Now you have A, the area is 1/2 bc sinA = 1/2 * 5x * 7x * sinA = 15*160