How do you factor #a^2-a-90#?

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Answer 1 · 2015-05-21T18:59:11

You find the roots and turn them into factors:

(Using Bhaskara here)

#(1+-sqrt(1-4(1)(90)))/2#
#(1+-19)/2#
Roots:
#a=10#, which can be rewritten as #(a-10)=0#
#a=-9#, which can be rewritten as #(a+9)=0#

Thus, #a^2-a-90=(a-10)(a+9)#

Answer 2 · 2015-05-21T21:01:53

A different method.

Problem: Factor #a^2-a-90#.

Find two numbers that when added equal -1, and when multiplied equal -90.

The numbers -10 and 9 fit the criteria.

#a^2-a-90=(a+9)(a-10)#