Find the roots

Algebra Level 2

Find the number of roots satisfying 3 x x 2 + x 2 = 0 3^x-x^2+x-2=0 .


The answer is 1.

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3 solutions

Kushal Bose
Sep 5, 2016

According to the question : 3 x = x 2 x + 2 3^x=x^2-x+2

The R.H.S. of the equation i.e. x 2 x + 2 x^2-x+2 has minimum value is 7 4 \frac{7}{4} which is at x = 1 / 2 x=1/2

At x = 1 / 2 x=1/2 the L.H.S. of the equation 3 x 3^x has value 1.732 1.732 which is lower than 7 / 4 7/4 .So, at x = 1 / 2 x=1/2 3 x 3^x is lower than x 2 x + 2 x^2-x+2 .

Taking the difference of their rate of increasing (i.e. derivative) 3 x l n 3 ( 2 x 1 ) 3^x ln3-(2 x-1)

It is easily seen that 3 x l n 3 ( 2 x 1 ) > 0 {3^x ln3-(2 x-1)} >0

At x = 1 / 2 x=1/2 3 x 3^x has alower value than x 2 x + 2 x^2-x+2 .But rate of increasing show that 3 x 3^x increase faster than x x + 2 x^-x+2 .

So, they must intersect at exactly one point and then increases.

So, there exists one solution.

Akshat Sharda
Sep 2, 2016

Make the graphs of y = 3 x y=3^{x} and y = x 2 x + 2 y=x^2-x+2 and check for the number of intersections.

But in exam hall? What to do there bro? No point in plotting graph.

Md Zuhair - 4 years, 9 months ago

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Nope. Plotting the graphs of these functions is no big deal, these type of problems can't be even done without graphs.

Akshat Sharda - 4 years, 9 months ago

So there's no way you can find x x without graphs ?

Jason Chrysoprase - 4 years, 9 months ago
Md Zuhair
Sep 1, 2016

@Chew-Seong Cheong please tell me the answer of this question. and the solution too.

@Calvin Lin Please help me do this sum . Dont delete it.

Md Zuhair - 4 years, 9 months ago

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