A right triangle has legs a and b , and hypotenuse c .
The length of the median to the hypotenuse is 3 a 3 b + b 3 a .
If H is the minimum possible value of c , find the value of 1 6 H .
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Well, I was going to post my method, but yours is way nicer than mine, so +1 from me!
Again , bashing!!
Let the vertex containing right angle is ( 0 , 0 ) . Others be ( a , 0 ) , ( 0 , b ) .
Now mid point of hypotenuse is ( a / 2 , b / 2 ) .
Using distance formulae and above given info,
c = 8 a b .
Or a 2 + b 2 = 6 4 ( a b ) 2 .
Now by A M − G M
a 2 + b 2 ≥ 2 ( a b ) .
Therefore a b ≥ 1 / 3 2 .
Hence c m i n = 1 / 4 = H .
Therefore answer is 4 .
Cosine rule for medium m delivers: m 2 = 4 c 2 + a 2 − 2 a 2 c c o s β = ( a 3 b + b 3 a ) 3 2
With c o s β = c a we get c 2 = a 2 + b 2 = 4 ( a 3 b + b 3 a ) 3 2
That means we have to minimize c 2 = a 2 + b 2 regarding constraint a 2 + b 2 = 4 ( a 3 b + b 3 a ) 3 2
This task is solved by the well known Lagrange multiplier method which relults in H=0.25.
What is the Lagrange multiplier method?How can it be applied to this?
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It is the determination of extrema for functions with more variables if these variables have to fulfill condition(s) like in the case given!
Is there any solution which does not require trigonometry?
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You can get 8ab =c, then apply AM-GM on a^2 + b^2, and put values in terms of c
How can you solve a problem avoiding the specific of it???
I have an elementary solution.
I'll post it when I am free.
Length of median from the vertex C of Δ A B C with opposite side length c and the other two side lengths a and b is given by
M = 2 1 2 a 2 + 2 b 2 − c 2
Considering the case of our right triangle, we have c 2 = a 2 + b 2 so that
M = 2 1 a 2 + b 2
Now,
⟹ ⟹ ⟹ ⟹ 2 1 a 2 + b 2 a 2 + b 2 6 a 2 + b 2 3 c c = = = = = 3 a 3 b + b 3 a 2 3 a b ⋅ 3 a 2 + b 2 2 3 a b 2 3 a b 8 a b
Using AM-GM inequality, we have
⟹ ⟹ ⟹ ⟹ ⟹ ⟹ 2 a 2 + b 2 2 c 2 2 6 4 a 2 b 2 3 2 a b a b 8 a b c ≥ ≥ ≥ ≥ ≥ ≥ ≥ a 2 b 2 a b a b 1 3 2 1 4 1 4 1
So the minimum value of c is H = 4 1 and thus 1 6 H = 4 .
Nice solution !
T h e m e d i u m = 2 1 c . F o r m i n i m u m c , l e g s m u s t b e e q u a l . S o a = b . a 2 + b 2 = 2 a 2 = c 2 . A n d a = 2 c . ⟹ 2 1 c = 3 a 3 b + a b 3 . ( 2 c ) 3 = 2 a 4 = 2 ( ( 2 ) 4 c ) . ∴ 8 c 3 = 2 ∗ 4 c 4 . ⟹ c = 4 1 = H . S o 1 6 ∗ H = 1 6 ∗ 4 1 = 4 .
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2 c = 3 a 3 b + b 3 a ⇔ 8 c 3 = ( a 2 + b 2 ) a b ⇔ c = 8 a b
Let θ be the angle between a and c . Then, c = 8 c 2 cos ( θ ) sin ( θ ) ⇔ c = 8 cos ( θ ) sin ( θ ) 1 ⇔ c = 4 sin ( 2 θ ) 1 . It has a maximum 4 1 when θ = 4 π .