It can be shown that probability p(r) that the electron in the ground state of the hydrogen atom will be detected inside a sphere of radius R is given by
`p(r)=1-e^(-2x)(1+2x+2x^(2))`.
in which x, a dimensionless quantity, is equal to `r//a`. Find r for `p(r)=0.90`.

Explanation

To solve the problem of finding \( r \) for \( p(r) = 0.90 \) in the context of the hydrogen atom, we will follow these steps:

### Step 1: Set up the equation
We start with the given probability equation:
\[ p(r) = 1 - e^{-2x}(1 + 2x + 2x^2) \]
where \( x = \frac{r}{a} \). We need to find \( r \) when \( p(r) = 0.90 \).
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