Maximizing Area

Geometry Level 1

What is the greatest possible area of a triangle with one side of length 7 and another of length 10?


The answer is 35.

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5 solutions

Archit Boobna
May 16, 2015

The figure shows this triangle with sides 10 10 , 7 7 and one unknown side.

We can see that the base of the triangle is 10 10 . And the height is 7 sin x 7\sin x .

We know that the area of a triangle is 1 2 b h \frac { 1 }{ 2 } bh . On putting values of b b and h h , we get that the area of this triangle is 1 2 ( 10 ) ( 7 sin x ) = 35 sin x \frac { 1 }{ 2 } \left( 10 \right) \left( 7\sin { x } \right) =35\sin { x }

To maximize this, we need to maximize sin x \sin x . We know that sin x \sin x lies from 1 -1 to 1 1 , so the maximum possible value is 1 1 .

So the answer is ( 35 ) ( 1 ) = 35 \left( 35 \right) \left( 1 \right) =\boxed { 35 }

Moderator note:

Great work. Bonus question: What would the maximum area of a quadrilateral with sides 7, 8, 9, 10?

This excellent proof demonstrates that a triangle area is greatest when the triangle is a right one, 'cause x equals 90.

Flávio Sallem - 4 years, 2 months ago
Danish Ahmed
May 14, 2015

The area A A of a triangle with sides a a , b b and included angle θ \theta is A = 1 2 a b sin θ A = \dfrac{1}{2}ab \sin \theta .

Then the area of the given triangle is 35 sin θ 35\sin{\theta} .

We need to maximize this expression from 0 < θ < π 0<\theta<\pi , and we see that sin θ \sin{\theta} is greatest when θ = π 2 \theta=\dfrac{\pi}{2} , so the maximum area is 35 sin π 2 = 35 35\sin{\dfrac{\pi}{2}}=\boxed{35}

Exactly how I did it, but I would have used 90 degrees since more people can understand and visualise degrees (at schoolboy level).

Sam Cheung - 6 years ago
Ryan Chua
May 25, 2015

The area of the triangle will be 1/2(10)(7)sinC

The maximum value of sinC is 1; the range of values for sinC go from -1 to +1

The maximum value of the area of the the triangle is 1/2(10)(7)(1) = 35

Tapan Roy
May 21, 2015

If base is 10, the area will be maximum when side with length 7 is perpendicular fto the base and thus the area is 1/2 x7x10= 35

Luke Pattison
May 17, 2015

(Half of the base) multiplied by height. Either 5x7=35 or 10x3.5=35

However, we are not given that those values are of the base and the height respectively. That is therefore an improper assumption.

Richard Ritz - 6 years ago

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However, using those side lengths we can assume on is our base. Lets say 10. If the other side length is 7 our height is >=7. Seven is the maximum value our height can be because if the angle between sides length 10 and 7 is less than or more than 90 degrees the height is less than 7.

Daniel Renkert - 6 years ago

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