1. Let f(x)=ax^2 +bx+a,where a and b are constants and a≠0. If one of the roots of the equation f(x)=0 is x=4, what is the other root?
2. Find the sum of the coefficients of the product (x^50+x^49+x^48+···+x^2+x+1)(x^50−x^49+x^48 −···+x^2 −x+1).
(In the second factor, the coefficients alternate between 1 and −1.)
Thank you.
Problem 1:
We can leverage the concept of sum and product of roots in quadratic equations.
Since x = 4 is a root, we know that f(4) = 0. This translates to:
16a + 4b + a = 0
Combining like terms, we get: 17a + 4b = 0 (Equation 1)
We are also given that f(x) = ax^2 + bx + a. Since a ≠ 0, we can rewrite f(x) by factoring out a:
f(x) = a(x^2 + bx/a + 1/a)
We know that one root is x = 4. This means that (x - 4) is a factor of f(x). Let's rewrite f(x) using this information:
f(x) = a(x - 4)(x + r) for some constant r (since the product of the roots is cr)
Equating both ways of expressing f(x), we get:
a(x^2 + bx/a + 1/a) = a(x - 4)(x + r)
Expanding both sides:
ax^2 + bx + a = ax^2 - (4a + r)x + 4ar
Equating the coefficients of like terms on both sides, we get:
b = - (4a + r) (Equation 2)
Now we have two independent equations (Equation 1 and Equation 2) with unknowns a, b, and r. We can use the fact that one root is x = 4 (reflected in Equation 1) to solve for the other root.
Since we're looking for the other root, let's try to eliminate a and b from the equations and solve for r.
From Equation 1: b = -17a/4
Substitute this into Equation 2:
-17a/4 = - (4a + r)
Solve for r:
17a/4 = 4a + r
r = 17a/4 - 4a
r = a/4
Now that we know r (product of roots), we can find the other root itself. Since the product of roots (cr) is also equal to f(4) (which is 0 in this case), we have:
(4)(other root) = 0
Since a ≠ 0, this implies the other root must be:
Other root = 0
In conclusion, the other root of the equation f(x) = 0 is x = 0.
Problem 2
Note that both factors share some common terms. Let's rewrite them to group these terms together:
(x^50 + x^48 + ... + x^2 + 1) + (x^49 - x^47 + ... + x - 1)
According to the sum of a finite geometric series formula, the sum of each individual sequence is:
(x^50 + x^48 + ... + x^2 + 1) = (x^(50)) * (1 / (1 - x^2)) = x^(50) / (1 - x^2)
(x^49 - x^47 + ... + x - 1) = (x^(49)) * (1 / (1 + x^2)) = x^(49) / (1 + x^2)
Now we can find the product:
Product = (x^(50) / (1 - x^2)) * (x^(49) / (1 + x^2))
Since we're only interested in the sum of the coefficients, we can ignore the constant terms in the denominators (they won't affect the sum). This simplifies the product to:
Product ≈ x^(50 + 49) // Ignoring constant terms in denominators
The sum of the coefficients in this product is simply the coefficient of the term with x raised to the highest power, which is x^(99). The coefficient of this term is 1.
In conclusion, the sum of the coefficients of the product is 1.