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# If g(x) = ax^5 + bx^3 + 1, and g(5) = 10, then g(–5) =?

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If g(x) = ax^5 + bx^3 + 1, and g(5) = 10, then g(–5) =? [#permalink]

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19 Aug 2013, 10:38
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If g(x) = ax^5 + bx^3 + 1, and g(5) = 10, then g(–5) =?

(A) – 10
(B) – 8
(C) – 6
(D) 0
(E) 4
[Reveal] Spoiler: OA

Last edited by Bunuel on 19 Aug 2013, 10:52, edited 1 time in total.
Edited the question and added the OA
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Re: If g(x) = ax5 + bx3 + 1 [#permalink]

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19 Aug 2013, 10:51
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Galiya wrote:
If g(x) = ax5 + bx3 + 1, and g(5) = 10, then g(–5) =?

(A) – 10
(B) – 8
(C) – 6
(D) 0
(E) 4

given:
$$g(5) = 10$$
therefore
$$a*5^5 + b5^3 + 1 = 10$$
$$a*5^5 + b5^3 = 9$$ ..................................(1
$$g(-5) = -a*5^5 - b5^3 + 1$$
$$=- (a*5^5 + b5^3)+1$$
now using 1
$$g(-5) = -9 + 1 = -8$$

hence B
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Re: If g(x) = ax^5 + bx^3 + 1, and g(5) = 10, then g(–5) =? [#permalink]

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19 Aug 2013, 12:39
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Galiya wrote:
If g(x) = ax^5 + bx^3 + 1, and g(5) = 10, then g(–5) =?

(A) – 10
(B) – 8
(C) – 6
(D) 0
(E) 4

g(5) = 10
or, 5^5 a + 5^3 b + 1 = 10
or, 5^5 a + 5^3 b = 9

g(-5) = -5^5 a - 5^3 b + 1 = - (5^5 a + 5^3 b) + 1 = - 9 + 1 = - 8 = (B)
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Re: If g(x) = ax^5 + bx^3 + 1, and g(5) = 10, then g(–5) =? [#permalink]

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13 Sep 2014, 21:45
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Re: If g(x) = ax^5 + bx^3 + 1, and g(5) = 10, then g(–5) =? [#permalink]

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04 Aug 2016, 21:02
Hello from the GMAT Club BumpBot!

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If g(x) = ax^5 + bx^3 + 1, and g(5) = 10, then g(–5) =? [#permalink]

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04 Aug 2016, 21:24
Galiya wrote:
If $$g(x) = ax^5 + bx^3 + 1$$, and $$g(5) = 10$$, then $$g(–5) =?$$

When raising a number to an odd power, if we negate the number, we negate the sign of the result.

$$ax^5 + bx^3 = 9 \, (x:5)\\ ax^5 + bx^3 = -9 \, (x:-5)$$

$$-9 + 1 = -8$$

[Reveal] Spoiler:
(B) – 8

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Re: If g(x) = ax^5 + bx^3 + 1, and g(5) = 10, then g(–5) =? [#permalink]

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07 Apr 2017, 14:23
it look me almost 3 minutes to solve it...
first:
g(5) = a*5^5 + b*5^3 +1 = 10
a*5^5 + b*5^3 = 9
5^3 * (a*5^2 + b) = 9
125 * (25a + b) = 9
25a + b = 9/125

now
g(-5) = a*-5^5 + b*-5^3 +1
we can factor out -5^3
-5^3 * (a*-5^2 +b) +1
-125*(25a + b) +1

we know 25a + b = 9/125
-125* 25a + b = -9
125*(25a + b) +1 = -8

Re: If g(x) = ax^5 + bx^3 + 1, and g(5) = 10, then g(–5) =?   [#permalink] 07 Apr 2017, 14:23
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# If g(x) = ax^5 + bx^3 + 1, and g(5) = 10, then g(–5) =?

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