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SajjadAhmad


On the xy-plane above, if the equation of \(l_{1}\) is \(y=\frac{1}{2}x\) and if point B is defined by the xy-coordinate pair (5,0), what is the area of OAB ?

(A) 4

(B) 3\(\sqrt{2}\)

(C) 2\(\sqrt{5}\)

(D) 5

(E) 7


The coordinates of the points A will be (2a,a) as y=x/2, and the other coordinates will be as shown.

Now, \(OA^2=(2a-0)^2+(a-0)^2=4a^2+a^2=5a^2\)
\(BA^2=(2a-5)^2+(a-0)^2=4a^2-20a+25+a^2=5a^2-20a+25\)
\(OB^2=5^2=25\)

By Pythagoras theorem,
\(OB^2=OA^2+AB^2\)
\(25=5a^2+5a^2-20a+25.......10a^2=20a....a=2\)

So if we take base as OB or 5, the height will be the y-coordinates of point A or a, that is 2.
Area = \(\frac{1}{2}*5*2=5\)

D
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AC/OC = 1/2 => OC = 2AC
BC/AC = 1/2 => AC = 2BC

=> OC = 2*2BC = 4BC
=> OC:BC = 4:1

=> AC = 2

Area = 0.5*2*5 = 5
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