Area of triangle!

Geometry Level pending

What is the area of the triangle formed by the lines 5 x 2 y = 10 , 5x-2y=10, 3 x + 4 y = 12 3x+4y=12 a n d and x a x i s x-axis ? Give answer till 2 digits after decimal point.


The answer is 1.15.

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2 solutions

Mahdi Raza
May 24, 2020

Solving two equations at a time, we get these three points of intersection: ( 2 , 0 ) , ( 4 , 0 ) , ( 32 13 , 15 13 ) (2,0), (4,0), (\frac{32}{13}, \frac{15}{13})

\[\begin{align} \text{Area }&= \frac{1}{2} \cdot b \cdot h \\ &= \frac{1}{2} \cdot(4-2)\cdot \bigg(\frac{15}{13} \bigg) \\ &= \frac{1}{\cancel{2}} \cdot(\cancel{2})\cdot \bigg(\frac{15}{13} \bigg) \\ &\approx \boxed{1.15}

\end{align}\]

Zakir Husain
May 24, 2020

To find the area you must know where the 3 points of the triangle lie:

  • One is at the intersection of x-axis and 5 x 2 y = 10 5x-2y=10
  • One is at the intersection of x-axis and 3 x + 4 y = 12 3x+4y=12
  • One is at the intersection of 5 x 2 y = 10 5x-2y=10 and 3 x + 4 y = 12 3x+4y=12

Any line a x + b y = c ax+by=c intersects x-axis at c a , 0 \frac{c}{a},0

Therefore 2 of the lines are ( 2 , 0 ) (2,0) and ( 4 , 0 ) (4,0)

The third line lies at the intersection of 5 x 2 y = 10 5x-2y=10 and 3 x + 4 y = 12 3x+4y=12 , that is it is the solution of these 2 equations

Solution of the equations: 3 x + 4 y = 12 3x+4y=12 5 x 2 y = 10 5x-2y=10 Multiplying our last equation by 2 10 x 4 y = 20 10x-4y=20 Adding this and our first equation 13 x = 32 13x=32 x = 32 13 \boxed{x=\frac{32}{13}} Put this in our first equation 3 ( 32 13 ) + 4 y = 12 3(\frac{32}{13})+4y=12 96 13 + 4 y = 12 \frac{96}{13}+4y=12 4 y = 12 96 13 4y=12-\frac{96}{13} y = 3 24 13 = 39 24 13 = 15 13 y=3-\frac{24}{13}=\frac{39-24}{13}=\boxed{\frac{15}{13}}

Now consider the line from ( 2 , 0 ) (2,0) to ( 4 , 0 ) (4,0) as the base of the triangle. Then the height will be the distance of ( 32 13 , 15 13 ) (\frac{32}{13},\frac{15}{13}) from the base and as the base is the x-axis the height is the ordinate of the point.

Therefore area= 1 2 ( 4 2 ) ( 15 13 ) = 15 13 = 1.15 384615... ) \frac{1}{2}(4-2)(\frac{15}{13})=\frac{15}{13}=\boxed{1.15}384615...)

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