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Problem using the bisection method (to be done on paper without matlab programming)

I was confused when I get f(xL)=0 times f(xr)=3.009636592, I get zero which suggest that xr is the answer. This obviously is wrong, so I would like to know what I am doing wrong. Thanks!

As depicted in the given figure, the velocity of water, v (m/s), discharged from a cylindrical tank

through a long pipe can be computed as

v = sqrt(2gH)*tanh (sqr(2gH)/2L)*t

where g = 9.81 m/s², H = initial head (m), L = pipe length (m), and t = elapsed time (s).

Note that tanh() is 'hyperbolic tangent' - you can use the 'tanh' function in your calculator or an online calculator to determine tanh(). Always use radians mode in calculating trigonometric functions in numerical methods.

Determine the head needed to achieve v = 5 m/s in 2.5 s for a 4-m-long pipe:

Find the head needed to achieve using the bisection method with the initial guesses x_l = 0 and x_u = 2 m with a stopping criterion of _s = 5%.

a. 1.4375

b. 1.460938

c. 1.46875 (I got this one and got it wrong)

d. 1.3750

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