Solving $\sin(x) + \ln(x) = 0$ without a calculator
algebra-precalculuslogarithmstrigonometry
How would you algebraically handle
$$\sin(x) + \ln(x) = 0$$
to find the zeroes without a graphing calculator?
Here's the graph
Is approximation your only hope? Can you get an exact symbolic answer?
Best Answer
Finding the zero algebraically without a graphing calculator is possible, but a symbolic answer is impossible (I believe), so only the algebraic part can be answered.
There are multiple ways of doing it. The most straightforward one to me is to cast it as a fixed-point iteration $x\to e^{-\sin(x)}$. The Mathematica command NestList[Exp[-Sin[#]] &, 0, 20] gives a sequence of numbers converging towards 0.57871. The first four items of the sequence are
Continuing from @calculus' comments, consider the function $$f(x)=x^2 + 2^x - 3$$ As noticed, $x=1$ is one solution.
On the other hand, $f(0)=-2$ and $f''(x)=2^x \log ^2(2)+2 \gt 0$; so there is another root. Using inspection $f(-1)=-\frac{3}{2}$, $f(-2)=\frac{5}{4}$ which shows a solution between $-2$ and $-1$.
Now, start Newton method, say with $x_0=-\frac{3}{2}$. This will produce as successive iterates $-1.64390$, $-1.63659$, $-1.63658$ which is the solution for six significant figures.
Best Answer
Finding the zero algebraically without a graphing calculator is possible, but a symbolic answer is impossible (I believe), so only the algebraic part can be answered.
There are multiple ways of doing it. The most straightforward one to me is to cast it as a fixed-point iteration $x\to e^{-\sin(x)}$. The Mathematica command
NestList[Exp[-Sin[#]] &, 0, 20]
gives a sequence of numbers converging towards 0.57871. The first four items of the sequence are$$ 1,e^{-\sin (1)},e^{-\sin \left(e^{-\sin (1)}\right)},e^{-\sin \left(e^{-\sin \left(e^{-\sin (1)}\right)}\right)} $$
Certainly, not all fixed-point iterations converge, but in this case, it does.