⎩ ⎪ ⎪ ⎨ ⎪ ⎪ ⎧ a + b + c = 1 a 2 + b 2 + c 2 = 2 a 3 + b 3 + c 3 = 3
Find a 5 + b 5 + c 5 .
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Note: It is easier to state that you are using Newton's identities, then (essentially) proving them.
Really a very nice and elegant approach.................. +1 upvote
Using Newton sum.
We get,
⇒
a
b
+
b
c
+
c
a
=
−
2
1
⇒ a b c = 6 1
⇒ a 4 + b 4 + c 4 = 6 2 5
And,
a 5 + b 5 + c 5 = ( a + b + c ) ( a 4 + b 4 + c 4 ) − ( a b + b c + c a ) ( a 3 + b 3 + c 3 ) + a b c ( a 2 + b 2 + c 2 )
a 5 + b 5 + c 5 = 6 2 5 + 2 3 + 3 1 = 6 3 6
∴ a 5 + b 5 + c 5 = 6
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p = a + b + c = 1 q = a b + b c + a c r = a b c
p 2 = 1 = ∑ a 2 + 2 ( a b + b c + a c ) ⟹ 2 + 2 q = 1 ⟹ q = − 2 1
f ( x ) = ( x − a ) ( x − b ) ( x − c ) = x 3 − p x 2 + q x − r = x 3 − x 2 − 2 x − r f ( a ) = f ( b ) = f ( c ) = 0
f ( a ) = 0 ⟹ a 3 − a 2 − 2 a − r = 0 ⟹ a 3 = a 2 + 2 a + r ∑ a 3 = 3 = ∑ a 2 + 2 ∑ a + 3 r = 2 5 + 3 r ⟹ 3 r = 2 1 ⟹ r = 6 1
a 3 = a 2 + 2 a + 6 1 ⟹ a 4 = a 3 + 2 a 2 + 6 a ∑ a 4 = ∑ a 3 + 2 ∑ a 2 + 6 ∑ a = 3 + 2 2 + 6 1 = 6 2 5
a 3 = a 2 + 2 a + 6 1 ⟹ a 5 = a 4 + 2 a 3 + 6 a 2 ∑ a 5 = ∑ a 4 + 2 ∑ a 3 + 6 ∑ a 2 = 6 2 5 + 2 3 + 6 2 = 6 3 6 = 6
a 5 + b 5 + c 5 = 6