Hilbert Spaces: Lebesgue integral introduced, step functions, 2-8-23 part 1

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The integral of step functions is well-defined do to the fact that it does not depend the representation of the step function. I feel a bit embarrasing asking this, but I have trouble approaching its proof. Could you provide some hint?

I guess a proof can start like this: let f be a step function and
p_1f_1+...+ p_nf_n,
q_1g1+...+q_mg_m
both be representations of $f$, where f_k, g_l are characteristic functions of [a_k, b_k), [a'_l, b'_l) respectively.
Need to show: p_1(b_1-a_1) + ... + p_n(b_n-a_n) = q_1(b'_1-a'_1) + ... + q_m(b'_l - a'_l)

Then I'm not sure how to proceed...

mingmiao