Pointing out the negativity....

Algebra Level 2

The only negative solution of the equation

2 x 2 x 1 1 = 2 x 1 + 1 2^{|x|} - |2^{x-1} - 1| = 2^{x-1} + 1

is:-


The answer is -1.

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1 solution

Abhishek Kanthed
Apr 22, 2014

Assuming that x<0, we replace, |x| by -x and -|2^{x-1} - 1| by 2^{x-1} - 1 as both x and |2^{x-1} - 1| are negative so the equation becomes,
2^{-x} + 2^{x-1} - 1 = 2^{x-1} + 1 So, it gives, 2^{-x} = 2 which implies that x=-1

Here's a more effecient method for when there are more moduli in the equation.

1.Equate the terms in the modulus to 0 and find the value of the variable. These will be the critical points. In this case, putting x = 0 x=0 and 2 x 1 1 = 0 2^{x-1}-1=0 The critical points will be 0 and 1.

2.Take all the possible cases. Substitute random values in the modulus according to the case and check whether the value of the whole modulus is positive or negative. If it is negative change the sign else don't change.For eg, when x < 0 x<0 2 x 2^{|x|} will become 2 x 2^{-x} and in 2 x 1 1 |2^{x-1}-1| Substituting x = -1, we get -0.75 and hence the sign should be changed. So for this case the final equation will become 2 x + 2 x 1 1 = 2 x 1 + 1 2^{-x}+2^{x-1}-1=2^{x-1}+1 Solving for x, we get x = 1 x=\boxed{-1} which lies in the range we took in the case, i.e. x<0.

3.Similarly for the cases 0 < = x < 1 0<=x<1 and x > = 1 x>=1 we get x = 1 x=1 and x > = 1 x>=1 But the 2nd case does not include 1 so ONLY FOR THAT CASE x =1 does'nt satisfy. So the complete answer is x = 1 U [ 1 , i n f i n i t y ) x={-1} U [1,infinity)

Vineeth Chelur - 7 years, 1 month ago

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Am.... That's what i meant... But thanks for clearification

Abhishek Kanthed - 7 years, 1 month ago

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