How do I factor #i^2-4t-21# completely?

1 Answer
Nghi N.
Jul 26, 2015

Factor:# t^2 - 4t - 21#

Ans: (x + 3)(x - 7)

Explanation:

f(t) = t^2 - 4t - 21
Find 2 numbers knowing sum (-4) and product (-21).
To find them, compose factor pairs of (-21), noticing that a and c have opposite signs-->(-1, 21)(-3, 7). This sum is 4 = -b . Then, the 2 numbers are 3 and -7.
y = (x + 3) (x - 7)

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