+0

# What are all real solutions to $2(2^x- 1) x^2 + (2^{x^2}-2)x = 2^{x+1} -2$ ?

0
70
1

$$2(2^x- 1) x^2 + (2^{x^2}-2)x = 2^{x+1} -2$$

Guest Jul 14, 2018
#1
+7002
+1

We need to solve $$2(2^x-1)x^2+(2^{x^2}-2)x-2^{x+1}-2=0$$.

Let $$f(x) = 2(2^x-1)x^2+(2^{x^2}-2)x-2^{x+1}-2$$.

If you graph f(x) using any method, you find that it only touches x-axis at one point.

We will use Newton-Rhapson to find solutions.

$$f'(x) = 4x(2^x-1)+2x^22^x\ln2+2^{x^2}-2+2x^2(2^{x^2}\ln2)-2^{x+1}\ln2$$

As this is too complicated, I solved it with a program.

Here is the source code in C++:
#include
using namespace std;
double f(double x){
return 2*(pow(2,x)-1)*x*x+(pow(2,x*x)-2)*x-pow(2,x+1)-2;
}
double f_prime(double x){
return 4*x*(pow(2,x)-1)+2*x*x*pow(2,x)*log(2)+pow(2,x*x)-2+2*x*x*(pow(2,x*x))*log(2)-pow(2,x+1)*log(2);
}
int main(){
double x = 1.1;
for (int i=0;i<1000;i++){ //iterate 1000 times
x -= f(x)/f_prime(x);
}
printf("Solution: %100g", x);
}

Output: Solution: 1.32006

MaxWong  Jul 15, 2018