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Algebra Difficulty 3.0 AMC 10/12 Find the answer

Given the equation x4+a1x3+a2x2+a3x+a4=(x+1)4+b1(x+1)3+b2(x+1)2+b3(x+1)+b4x^{4}+a_{1}x^{3}+a_{2}x^{2}+a_{3}x+a_{4}=(x+1)^{4}+b_{1}(x+1)^{3}+b_{2}(x+1)^{2}+b_{3}(x+1)+b_{4}, define the mapping f(a1,a2,a3,a4)=b1b2+b3b4f(a_{1},a_{2},a_{3},a_{4})=b_{1}-b_{2}+b_{3}-b_{4}. Find the value of f(2,0,1,6)f(2,0,1,6).

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Solution

From the given equation, we have x4+a1x3+a2x2+a3x+a4=(x+1)4+b1(x+1)3+b2(x+1)2+b3(x+1)+b4x^{4}+a_{1}x^{3}+a_{2}x^{2}+a_{3}x+a_{4}=(x+1)^{4}+b_{1}(x+1)^{3}+b_{2}(x+1)^{2}+b_{3}(x+1)+b_{4}.

Substitute a1=2a_{1}=2, a2=0a_{2}=0, a3=1a_{3}=1, and a4=6a_{4}=6 into the equation, we get x4+2x3+x+6=(x+1)4+b1(x+1)3+b2(x+1)2+b3(x+1)+b4x^{4}+2x^{3}+x+6=(x+1)^{4}+b_{1}(x+1)^{3}+b_{2}(x+1)^{2}+b_{3}(x+1)+b_{4}.

Now, let's find the value of f(2,0,1,6)f(2,0,1,6), which is b1b2+b3b4b_{1}-b_{2}+b_{3}-b_{4}. To do this, we need to find the values of b1b_{1}, b2b_{2}, b3b_{3}, and b4b_{4}.

Let's take x=2x=-2 in the equation x4+2x3+x+6=(x+1)4+b1(x+1)3+b2(x+1)2+b3(x+1)+b4x^{4}+2x^{3}+x+6=(x+1)^{4}+b_{1}(x+1)^{3}+b_{2}(x+1)^{2}+b_{3}(x+1)+b_{4}. This simplifies to b1b2+b3b4=3b_{1}-b_{2}+b_{3}-b_{4}=-3.

Therefore, the value of f(2,0,1,6)f(2,0,1,6) is 3\boxed{-3}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.