Strange Error Message when Plotting with a LaTeX Legend

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The following code is from the 1st edition of my textbook, Sage for Undergraduates. It is producing some extremely unexpected errors (technically, deprication warnings). Something about an invalid escape sequence, \i, which I'm not using.

Can someone explain how I should change my code, so that these warnings do not appear?

Code:

P1= text('$\int_0^2 \sqrt{x} \, dx$', (0.5, 1.1), fontsize=20)

P2= plot( sqrt(x), 0, 2, fill=True)

P= P1 + P2

P.show()

Warning: <input>:1: DeprecationWarning: invalid escape sequence \i

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