Difference between revisions of "1983 AHSME Problems/Problem 18"
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− | ... | + | ==Problem== |
+ | |||
+ | Let <math>f</math> be a polynomial function such that, for all real <math>x</math>, | ||
+ | <math>f(x^2 + 1) = x^4 + 5x^2 + 3</math>. | ||
+ | For all real <math>x, f(x^2-1)</math> is | ||
+ | |||
+ | <math>\textbf{(A)}\ x^4+5x^2+1\qquad | ||
+ | \textbf{(B)}\ x^4+x^2-3\qquad | ||
+ | \textbf{(C)}\ x^4-5x^2+1\qquad | ||
+ | \textbf{(D)}\ x^4+x^2+3\qquad | ||
+ | \textbf{(E)}\ \text{none of these} </math> | ||
+ | |||
+ | ==Solution== | ||
+ | |||
+ | Let <math>y = x^2 + 1</math>. Then <math>x^2 = y - 1</math>, so we can write the given equation as | ||
+ | <cmath>\begin{align*}f(y) &= x^4 + 5x^2 + 3 \\ | ||
+ | &= (x^2)^2 + 5x^2 + 3 \\ | ||
+ | &= (y - 1)^2 + 5(y - 1) + 3 \\ | ||
+ | &= y^2 - 2y + 1 + 5y - 5 + 3 \\ | ||
+ | &= y^2 + 3y - 1.\end{align*}</cmath> | ||
+ | Then substituting <math>x^2 - 1</math> for <math>y</math>, we get | ||
+ | <cmath>\begin{align*}f(x^2 - 1) &= (x^2 - 1)^2 + 3(x^2 - 1) - 1 \\ | ||
+ | &= x^4 - 2x^2 + 1 + 3x^2 - 3 - 1 \\ | ||
+ | &= x^4 + x^2 - 3.\end{align*}</cmath> | ||
+ | The answer is therefore <math>\boxed{\textbf{(B)}}</math>. | ||
+ | |||
+ | ==See Also== | ||
+ | {{AHSME box|year=1983|num-b=17|num-a=19}} | ||
+ | |||
+ | {{MAA Notice}} |
Revision as of 00:55, 20 February 2019
Problem
Let be a polynomial function such that, for all real , . For all real is
Solution
Let . Then , so we can write the given equation as Then substituting for , we get The answer is therefore .
See Also
1983 AHSME (Problems • Answer Key • Resources) | ||
Preceded by Problem 17 |
Followed by Problem 19 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 • 26 • 27 • 28 • 29 • 30 | ||
All AHSME Problems and Solutions |
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