What is the greatest possible area of a triangle with one side of length 7 and another of length 10?
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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.
The area A of a triangle with sides a , b and included angle θ is A = 2 1 a b sin θ .
Then the area of the given triangle is 3 5 sin θ .
We need to maximize this expression from 0 < θ < π , and we see that sin θ is greatest when θ = 2 π , so the maximum area is 3 5 sin 2 π = 3 5
Exactly how I did it, but I would have used 90 degrees since more people can understand and visualise degrees (at schoolboy level).
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
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
(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.
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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.
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The figure shows this triangle with sides 1 0 , 7 and one unknown side.
We can see that the base of the triangle is 1 0 . And the height is 7 sin x .
We know that the area of a triangle is 2 1 b h . On putting values of b and h , we get that the area of this triangle is 2 1 ( 1 0 ) ( 7 sin x ) = 3 5 sin x
To maximize this, we need to maximize sin x . We know that sin x lies from − 1 to 1 , so the maximum possible value is 1 .
So the answer is ( 3 5 ) ( 1 ) = 3 5