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# How do you know when to handle all three coefficients? or only two in factoring?

Sometimes im told to find 2 numbers that add to 6 and multiply to 8 or I need to find the gcf or I need to find number for all the numbers and the varible too? Like how do you know what to do in what problem? Some start with x^2 some start with 8x^2, its confusing.

### 2 Answers by Expert Tutors

Parviz F. | Mathematics professor at Community CollegesMathematics professor at Community Colle...
4.8 4.8 (4 lesson ratings) (4)
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X^ 2 + 7 X + 10  =0

always    f(x) = ( X +a ) ( X+b ) = X^2 + ( a +b ) X + ab

Roots , the values of X that makes f( x) =0

(  X+a ) = 0   X1 = -a     ( X+ b) = 0   X2 = -b

X^2 + ( a + b) X + ab =  X^2 + ( X1 + X2 ) X + X1X2

with a =1

always :   X1 + X2 = - b
X1 . X 2 = c

X^2 + 7X + 10 =0
- (X1 + X2) =  7     X1 . X2 = 10  ( 2)

2, 5 are the answer for X1 x2
then :

X^2 + 7X + 10  = ( X +2 ) ( X +5 ) =0   X = -2   X = -5

If  System of equation( 2 ) doesn't have integer solution, then we have to factor by completing

the square.

for a ≠ 1

i.e.
2 X^2 + 7X +5 = 0        then -( X 1 + X2 )= 7/2     X1 . X2 =  ( 5 /2)

again answer X1 = -1  X2 =- 5/2
To factor we break  7 into 2 +5

2 X^2 + 2X + 5X + 5=0

Factoring by grouping :

2X ( X + 1 ) + 5 ( X +1 ) = 0

( X +1 ) ( 2X + 5) =0

X = -1   X = -5/2

Steve S. | Tutoring in Precalculus, Trig, and Differential CalculusTutoring in Precalculus, Trig, and Diffe...
5.0 5.0 (3 lesson ratings) (3)
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Life is full of different problems.

Based on our past experiences we try to predict what will happen in the future.

We sometimes do that with math models.

And since they are different problems the models will be different.

The study of mathematics is aimed at creating math models for each of the different problems and then solving the models to predict real life.