If x 2 − x − 1 = 0 , then find the value of x 3 − 2 x + 1 .
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Sir, I suppose there wouldn't be a square sign in the last line while replacing for x^3..
Could you also please solve my problem on the link
Sir, please me with the problem on this link. https://brilliant.org/discussions/thread/algebra-mania/
Let
x
3
−
2
x
+
1
=
S
Then;
x
3
+
1
=
S
+
2
x
⟹
(
x
+
1
)
(
x
2
−
x
+
1
)
=
S
+
2
x
(Since,
a
3
+
b
3
=
(
a
+
b
)
(
a
2
−
a
b
+
b
2
)
)
⟹
2
(
x
+
1
)
=
S
+
2
x
(Since,
x
2
−
x
−
1
=
0
;
=
=
>
x
2
−
x
+
1
=
2
)
⟹
2
x
+
2
=
S
+
2
x
Hence,
x
3
−
2
x
+
1
=
S
=
2
Try this question!! https://brilliant.org/discussions/thread/algebra-mania/
x 3 − 2 x + 1 = ( x 2 − x − 1 ) ( x + 1 ) + 2
= ( 0 ) ( x + 1 ) + 2
= 2
Please help me in solving this problem, https://brilliant.org/discussions/thread/algebra-mania/
The given expression can be be rearranged as
x 3 − 2 x − 1 = x ( x 2 − 2 ) + 1 = x ( x + 1 − 2 ) + 1 as x 2 − x − 1 = 0 ⇒ x 2 = x + 1
= x 2 − x + 1 = x 2 − x − 1 + 2 = 2 as x 2 − x − 1 = 0
Hence the answer is 2.
x 3 − 2 x + 1 = ( x 3 + 1 ) − 2 x = [ ( x + 1 ) ( x 2 − x − 1 ) + 2 x + 2 ] − 2 x = [ ( x + 1 ) ⋅ 0 + 2 x + 2 ] − 2 x = 0 + 2 x + 2 − 2 x = 2
x ² − x − 1 = 0 . . . . . . . . . . ( 1 )
x ² = x + 1
Multiply both side with x
x ³ = x ² + x . . . . . . . . . . ( 2 )
Otherwise, from ( 1 ) we get
x = x ² − 1
Adding x on both sides we get
2 x = x ² + x − 1 . . . . . . . . . . ( 3 )
Now,
x ³ − 2 x + 1 = ( 2 ) − ( 3 ) + 1
= x ² + x − ( x ² + x − 1 ) + 1
= x ² + x − x ² − x + 1 + 1
= 2
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It is given that x 2 − x − 1 = 0 ⟹ x 2 = x + 1 ⟹ x 3 = x 2 + x = ( x + 1 ) + x = 2 x + 1 .
Therefore, x 3 − 2 x + 1 = ( 2 x + 1 ) − 2 x + 1 = 2 .