Exercise 2: Assembling the Full Chain for dL/db1 — Possible Solution
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GIVEN
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dL/dz2 = -2.5538, w2 = -0.8, a1 = 0.7211

STEP 1: COMPUTE da1/dz1 USING CHAPTER 4'S SIGMOID DERIVATIVE
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Chapter 4's own formula: sigma'(z) = sigma(z)*(1-sigma(z))

Since a1 = sigma(z1), this is just:
da1/dz1 = a1*(1-a1) = 0.7211*(1-0.7211) = 0.7211*0.2789 = 0.2011

STEP 2: ASSEMBLE EVERY LINK OF THE CHAIN
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dL/db1 = dL/dz2 * dz2/da1 * da1/dz1 * dz1/db1

dz2/da1 = w2 = -0.8 (from z2 = w2*a1 + b2)
da1/dz1 = 0.2011 (computed above)
dz1/db1 = 1 (from z1 = w1*x + b1, the derivative with respect to
  b1 is just 1)

STEP 3: MULTIPLY EVERY LINK TOGETHER
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dL/db1 = (-2.5538) * (-0.8) * (0.2011) * (1)

First: (-2.5538) * (-0.8) = 2.0430
Then: 2.0430 * 0.2011 = 0.4109
Then: 0.4109 * 1 = 0.4109

RESULT
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dL/db1 = 0.4109 (matching this chapter's own stated value)

WHY THIS WORKS AS AN ANSWER
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da1/dz1 is computed by explicitly reusing Chapter 4's own sigmoid
derivative formula (not re-derived from scratch), and every one of
the four links in the chain is multiplied together in sequence,
showing the running product at each step, rather than jumping
straight to the final answer.