How do you find the zeros of #f(x) = x^3 + 4x^2 - 25x - 100#?
2 Answers
There are three zeros:
Explanation:
You should try and recognize patterns in your expression that would help you factorize.
For example, here, you can notice that
# x^3 + 4x^2 - 25x - 100 = x^2(x+4) - 25(x+4) = (x^2 - 25)(x+4) #
Now you can use the identity
#... = (x+5)(x - 5)(x+4) #
Now,
#f(x) = 0#
#<=> x^3 + 4x^2 - 25x - 100 = 0#
#<=> (x+5)(x-5)(x+4) = 0#
A product is equal to zero if one or more factors is/are equal to zero:
#<=> x+5 = 0 " or " x-5 = 0 " or " x+4 = 0#
#<=> x = -5 " or " x = 5 " or " x = -4#
Explanation:
Factor by grouping:
#x^3+4x^2-25x-100#
#=(x^3+4x^2)-(25x+100)#
#=x^2(x+4)-25(x+4)#
#=(x^2-25)(x+4)#
#=(x+5)(x-5)(x+4)#
This gives the solutions
#{(x=-5),(x=5),(x=-4):}#