Questions   
Sort: 
 #1
avatar+129840 
+2

We  can find  the height  of  this triangle  as  follows

 

Using  Heron's Formula  we have  the area as

sqrt  (9 (9-8) (9-6) (9 - 4))  = sqrt  ( 9 * 3 * 5)  =  3sqrt(15)

So.....to find the height we  have

Area  = (1/2)(BC) height

3sqrt (15)  =  (1/2)(4) height

3sqrt (15)  = 2 * height

(3/2)sqrt (15)  = 1.5 sqrt (15)  =  height  = y coordinate of A

 

And we  can find  the  x coordinate  of A  by the Pythagorean Theorem

 

sqrt  [ AC^2  -  height of ABC^2 ]   = sqrt  (8^2   -(1.5 sqrt (15))^2  )  = sqrt (30.25)  =  5.5

 

So....the coordinates  of A  = (5.5, 1.5sqrt (15))

 

 

We  can use a formula to find  the coordinates  of  the center  of the incircle

Let B = (4,0)  and C  = (4,0)

 

x coordinate  of incenter  =

 

[ Ax* a  + Bx* b  + Cx * c ]  / perimeter

 

Where Ax = the x coordinate  of A  Bx  = x coordinate of B    Cx  = x coordinate of C

And a, b , c  are  the sides lengths opposite A, B and C

 

So  we have

 

[ 5.5 (4)  + (4)(8)  + (0)(6)] / 18  =  3

 

Similarly the  y coordinate  of the center of  the incircle  =  

[Ay * a  + By * b + Cy * c ]  / 18 

 

[ 1.5sqrt (15)(4)  + (0)(8) + (0)(6)  / 18   =  sqrt (15)/3   =    radius  of incircle

 

Then the  height of triangle ANM = height of triangle ABC  - 2* incircle radius = sqrt (15)  ( 3/2 - 2/3)  =

 

(5/6) sqrt (15 )

 

And since triangles  AMN and ABC  are similar

 

Then

 

MN  / height of AMN  =  BC / height of ABC

 

MN  / [ (5/6)sqrt (15) ] =   4 /  [  (3/2 ) sqrt (15) ]

 

MN =  4 (5/6)  / (3/2)   =  4 ( 5/6) (2/3)  =  40 /18  =  20/9  

 

 

cool cool cool

May 22, 2020
 #2
avatar+23251 
0

A formula for (a + b)3  =  a3 + 3·a2·b + a·b2 + b3.

[You can check this formula by multiplication of (a+b)·(a + b)·(a + b).]

 

For  (n - 1)3  by using the above formula  --->   a = n  and  b = -1:

(a + b)3  =  a3   + 3·a2·b       + 3·a·b2       + b3.

(n - 1)3  =  (n)3 + 3·(n)2·(-1) + 3·(n)·(-1)2 + (-1)3.

             =  n3 - 3n2 + 3n - 1

 

For  (n - 2)3  by using the above formula  --->   a = n  and  b = -2:

(a + b)3  =  a3   + 3·a2·b       + 3·a·b2       + b3.

(n - 1)3  =  (n)3 + 3·(n)2·(-2) + 3·(n)·(-2)2 + (-2)3.

             =  n3 - 6n2 +12n - 8

 

For  (n - 3)3  by using the above formula  --->   a = n  and  b = -3:

(a + b)3  =  a3   + 3·a2·b       + 3·a·b2       + b3.

(n - 3)3  =  (n)3 + 3·(n)2·(-3) + 3·(n)·(-3)2 + (-3)3.

             =  n3 - 9n2 +27n - 27

 

Putting all this together:

 

n3  =  (n - 1)3 + (n - 2)3 + (n - 3)3

n3  =  ( n3 - 3n2 + 3n - 1 ) + ( n3 - 6n2 +12n - 8 ) + ( n3 - 9n2 +27n - 27 )

n3  =  3n3 -18n2 + 42n - 36

  0  =  2n3 - 18n2 + 42n - 36

 

This is not nice ... so, if there is an integer value for a solution, it must be a factor of 36 ...

keep trying them (1, -1, 2, -2, 3, -3, 4, -4, 6, -6, 9, -9, 18, -18, 36, -36) in the problem

until you find one that works ... fortunately, there is one:  n  =  6.

 

Factor out the (n - 6) factor to get  (n - 6)(2n- 6n + 6) =  2(n - 6)(n2 - 3n + 3)

 

n2 - 3n + 3  has no integer solutions (it doesn't even have real solutions),

so the only integer solution is 6.

May 22, 2020

5 Online Users

avatar
avatar