SOLUTION: Form a​ fifth-degree polynomial function with real coefficients such that 2i,1-2i, and -5 are zeros and f(0)=200.

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Question 1126935: Form a​ fifth-degree polynomial function with real coefficients such that 2i,1-2i, and -5 are zeros and f(0)=200.
Found 2 solutions by MathLover1, solver91311:
Answer by MathLover1(20850) About Me  (Show Source):
You can put this solution on YOUR website!

given:
x%5B1%5D=2i, if that zero given, the conjugate x%5B2%5D=-2i must also be a zero
x%5B3%5D=1-2i, we also have x%5B4%5D=1%2B2i
recall: complex zeros always come in pairs
x%5B5%5D=+-5+
+f%280%29=200

f%28x%29=C%28x-x%5B1%5D%29%28x-x%5B2%5D%29%28x-x%5B3%5D%29%28x-x%5B4%5D%29%28x-x%5B5%5D%29 where C is a constant, we put this for the y-intercept condition
f%28x%29=C%28x-2i%29%28x-%28-2i%29%29%28x-%281-2i%29%29%28x-%281%2B2i%29%29%28x-%28-5%29%29+
f%28x%29=C%28x-2i%29%28x%2B2i%29%28x-1%2B2i%29%28x-1-2i%29%28x%2B5%29
f%28x%29=C%28x%5E2-4i%5E2%29%28x%5E2+-+2+x+%2B+5%29%28x%2B5%29+
f%28x%29=C%28x%5E2-4%28-1%29%29%28x%5E3+%2B+3+x%5E2+-+5+x+%2B+25%29+
f%28x%29=C%28x%5E2%2B4%29%28x%5E3+%2B+3+x%5E2+-+5+x+%2B+25%29
f%28x%29=C%28x%5E5+%2B+3+x%5E4+-+x%5E3+%2B+37+x%5E2+-+20+x+%2B+100+%29

Using the fact that f%280%29=200, we can solve for C.

200+=+C+%280%5E5+%2B+3+%2A0%5E4+-+0%5E3+%2B+37%2A0%5E2+-+20+%2A0+%2B+100+%29

200+=+100C
C=200%2F100
C=2
so, we have
f%28x%29=2%28x%5E5+%2B+3+x%5E4+-+x%5E3+%2B+37+x%5E2+-+20+x+%2B+100+%29
f%28x%29=2x%5E5+%2B+6x%5E4+-+2x%5E3+%2B+74x%5E2+-+40+x+%2B+200+



Answer by solver91311(24713) About Me  (Show Source):
You can put this solution on YOUR website!


Complex zeros always appear in conjugate pairs, thus if is a zero, then is also a zero. Hence the 5 zeros of your desired polynomial are: and .

If is a zero of a polynomial function , then is a factor of the polynomial. So, in factored form, your polynomial is:



Multiplying it out is left as an exercise for the student.


John

My calculator said it, I believe it, that settles it