SOLUTION: Suppose f(x)=ax^4+bx^2+x+5, where a and b are constants. Find f(4) if f(-4)=3

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Question 995514: Suppose f(x)=ax^4+bx^2+x+5, where a and b are constants. Find f(4) if f(-4)=3
Answer by ikleyn(52785) About Me  (Show Source):
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Suppose f(x)=ax^4+bx^2+x+5, where a and b are constants. Find f(4) if f(-4)=3.
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Let us write the fact that  f(-4) = 3.
Simply substitute  x = -4  into the given polynomial.  You will have

f(-4) = a%2A%28-4%29%5E4+%2B+b%2A%28-4%29%5E2+%2B+%28-4%29+%2B+5 = 3.

Simplify it,  taking into account that  %28-4%29%5E4 = 4%5E4  and  %28-4%29%5E2 = 4%5E2.  You will have

f(-4) = a%2A4%5E4+%2B+b%2A4%5E2+%2B+%28-4%29+%2B+5 = 3.         (1)

Now express  f(4).  Simply substitute  x = 4  into the given polynomial.  You will get

f(4) = a%2A4%5E4+%2B+b%2A4%5E2+%2B+4+%2B+5.                     (2)

Now compare expressions  (1)  and  (2).  They are almost identical.  The difference is in linear terms  (-4)  in   (1)  and  4  in  (2).
Hence,  f(4)  is in  8  units greater than  f(-4).  It means that  f(4) = f(-4) + 8 = 3 + 8 = 11.

Answer.  f(4) = 11.