Wednesday, August 9, 2017

99-GRE Math Test - Important Key points and formulas Part - 4

Blog-99

In continuation of Blog-98, we will see all the important formulas and useful statements which are to be used in Math test of GRE.

Factorization (Continued):

b) Factorization of Polynomial (Continued)

2) (x + a) (x - b) = x (x - b) + a (x - b)
                          = x 2 - b x + a x - ab
                          = x 2 + a x - b x - ab
                          = x 2 + (a - b) x - ab
Generally we call x 2 as the first term, (a + b) x as the middle term and ab as the last term. 

Basic concept: 

a) Step-1: See the sign of the last term.
b) Step-2: Here it is "-" so factorize the product of the coefficient of first term (here it is 1) and the last term in such a way that the DIFFERENCE of these two factors must be the coefficient of the middle term.
c) Step-3: Get the factors.

Example-1:

Factorize: x 2 + 4 x - 21.

a) Step-1: Here sign of the last term 21 is "-"
b) Step-2: The coefficient of the first term is 1 and the last term is 21 , so the product of 1 and 21 is 21. Now the factors of 21 are 3 and 7 and as the sign of the last term is "-", their subtraction is 7 - 3 = 4 which is the coefficient of the middle term. (Note: Here, the coefficient of the middle term is positive so we took it as 7 - 3).  

                         = x 2 + 4 x - 21
                         = x 2 + (7 - 3) x - (3 x 7)
                         = x 2 + 7 x - 3 x - (3 x 7)
                         = x (x + 7) - 3 (x + 7)
                         = (x - 3) (x + 7)
c) Step-3: So the factors of x 2 + 4 x - 21 are (x - 3) and (x + 7) 

Example-2:

Factorize: 8 x 2 + 18 x - 5.

a) Step-1: Here sign of the last term 5 is "-"
b) Step-2: The coefficient of the first term is 8 and the last term is 5 , so the product of 8 and 5 is 8 X 5. Now the factors of 8 X 5 are 2, 2, 2 and 5 and as the sign of the last term is "-", so, we take two factor in such a way that their difference will be 18. Here 2, 2, 2 and 5 will give us 2 and 20. So, here subtraction is 20 - 2 = 18 which is the coefficient of the middle term.  

                         = 8 x 2 + 18 x - 5
                         = 8 x 2 + (20 - 2) x - 5
                         = 8 x 2 + 20 x - 2 x - 5
                         = 4 x (2 x + 5) - 1 (2 x + 5)
                         = (2 x + 5) (4 x - 1) 
c) Step-3: So the factors of 8 x 2 + 18 x + 5 are (2 x + 5) and (4 x - 1).

In the next part we will see remaining 2 types in detail. These 2 types are given below.

3) (x - a) (x + b) = x 2 - (a - b) x - ab
4) (x - a) (x - b) = x 2 - (a + b) x + ab