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Question


Find the complete range of values of x that satisfies |x+3|>x211x+30.


Options:
A .   (,3)(3,)
B .   (-9, 3)
C .   (,3)(9,)
D .   (3, 9)
E .   B at 1pm
Answer: Option D
:
D

Conventional Approach :
Given inequality is |x+3|>x211x+30
We consider 2 cases 
Case (i) : x>3
|x+3|=x+3
x+3>x211x+30x212x+27<0
(x9)(x3)<0
 x= (3,9)
Case (ii) ; x<3
|x+3|= -x-3
x3>x211x+30
x210x+33<0
x210x+25+8<0=(x5)2+8<0
(x5)2+8 is always a positive quantity, irrespective of the value of x. Hence we do not have any value for x in the interval. Hence  the valid interval for x is (3,9). 


Shortcut- Elimination Approach


Step 1: Choose a value for x which is present in 2 answer options and absent in the other 2. Eg) x=4 satisfies the range specified in option (a) and (d), but does not form a part of the range in option (b) and (c)


At x=4


LHS= |4+3|=7


RHS= 16 - 44+30 = 2. LHS>RHS. It is satisfied. This means that answer options, which do not include x=4 can be directly eliminated as they can NEVER be the right answer


Step 2- Choose a value for x which is there in one of the remaining 2 options and absent in the other


Eg) Take x=10. This option satisfies the range in option (a) but is absent in option (d)


At x=10


LHS= |10+3|=13


RHS= 100-110+30= 20


LHS<RHS. This violates the condition in the question. This means that option (a) can never be the right answer.


Thus, by elimination, answer is option (d) 



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