If a + b + c = 6 , a 2 + b 2 + c 2 = 1 4 and a 3 + b 3 + c 3 = 3 6 , find the value of
a 4 + b 4 + c 4 .
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Who would have thought?!!
I solved it the same way!!
a 2 + b 2 + c 2 = ( a + b + c ) 2 − 2 ( a b + a c + b c ) = ( 6 ) 2 − 2 ( a b + a c + b c ) = 1 4 → ( a b + a c + b c ) = 1 1 a 3 + b 3 + c 3 = ( a + b + c ) 3 − 3 [ ( a + b + c ) ( a b + a c + b c ) − a b c ] = ( 6 ) 3 − 3 [ ( 6 ) ( 1 1 ) − a b c ] = 3 6 → ( a b c ) = 6 a 4 + b 4 + c 4 = ( a + b + c ) 4 − 1 2 ( a b c ) ( a + b + c ) − 4 [ a 3 ( c + b ) + b 3 ( c + a ) + c 3 ( a + b ) ] − 6 [ ( a b + a c + b c ) 2 − 2 a b c ( a + b + c ) ] a 4 + b 4 + c 4 = ( 6 ) 4 − 1 2 ( 6 ) ( 6 ) − 4 [ a 3 ( 6 − a ) + b 3 ( 6 − b ) + c 3 ( 6 − c ) ] − 6 [ ( 1 1 ) 2 − 2 ( 6 ) ( 6 ) ] a 4 + b 4 + c 4 = 1 2 9 6 − 4 3 2 − 4 [ ( 6 a 3 − a 4 + 6 b 3 − b 4 + 6 c 3 − c 4 ) ] − 2 9 4 = a 4 + b 4 + c 4 = 1 2 9 6 − 4 3 2 − 4 [ 6 ( a 3 + b 3 + c 3 ) − ( a 4 + b 4 + c 4 ) ] − 2 9 4 = a 4 + b 4 + c 4 = 1 2 9 6 − 4 3 2 − 4 [ 6 ( 3 6 ) − ( a 4 + b 4 + c 4 ) ] − 2 9 4 = a 4 + b 4 + c 4 = 1 2 9 6 − 4 3 2 − 8 6 4 + 4 ( a 4 + b 4 + c 4 ) − 2 9 4 = a 4 + b 4 + c 4 = 4 ( a 4 + b 4 + c 4 ) − 2 9 4 = → ( a 4 + b 4 + c 4 ) = 3 2 9 4 = 9 8
Complicated
awesome work
Cool, but it's messy. Maybe Newton's sums would be a better option
did exactly the same way
It can be guessed pretty easily
I prefer to use Vieta's formula and Newton's sums to do this.
From Vieta's formula, we have:
a + b + c = 6
a b + a c + b c = x
a b c = y
To find a 4 + b 4 + c 4 , we will need to find x and y first.
Using Newton's sums:
a 2 + b 2 + c 2 = ( a + b + c ) ( a + b + c ) − 2 ( a b + a c + b c )
1 4 = 6 ( 6 ) − 2 x ⟹ x = 1 1 ⟹ a b + a c + b c = 1 1
Next,
a 3 + b 3 + c 3 = ( a + b + c ) ( a 2 + b 2 + c 2 ) − ( a b + a c + b c ) ( a + b + c ) − 3 a b c
3 6 = 6 ( 1 4 ) − 1 1 ( 6 ) − 3 y ⟹ y = − 6 ⟹ a b c = − 6
Then, substitute in here to find the answer:
a 4 + b 4 + c 4 = ( a + b + c ) ( a 3 + b 3 + c 3 ) − ( a b + a c + b c ) ( a 2 + b 2 + c 2 ) − a b c ( a + b + c ) = 6 ( 3 6 ) − 1 1 ( 1 4 ) − ( − 6 ) ( 6 ) = 9 8
Note: Is this really a level 1 Algebra question?
I think that there is a slight mistake in the solution presented although the result is correct. It should be y=6, because the factorization of the terms a^3+b^3+c^3 should end with +3ab. The same mistake is in the next factorization so the error cancels out
1+2+3 = 6 and they cannot be the same number as 2+2+2 so I used this method 1+2+3 and squared it , cubed it and I did it to the power of 4
BTW i am only 10 yrs old
Problem Loading...
Note Loading...
Set Loading...
(This is not a recommended approach to the problem...)
"Hmm...I wonder if they are integers..."
"Well, what about 1, 2, and 3?"
"Oh, hmm, those work. Cool."
1 4 + 2 4 + 3 4 = 1 + 1 6 + 8 1 = 9 8 .