How to do the first question of the ability challenge on page 46 of the first volume of the eighth grade mathematics synchronous training?

Because ∠ c = 90.

So what? ∠A+∠B=90

In a triangle? At ACD? Because AC=AD? So what? ∠ACD equals∠ ∠ADC.

∠A plus ∠ACD And ∠ADC equals 180.

∠ ADC = (180-∠ A) divided by 2

Same reason? ∠ CEB = (180-∠ b) divided by 2.

So what? In the triangle, CED ∠ ECD = 180-∠ CED+∠ EDC.

∠ ECD = 180-(90+ 1/2 ∠A？ +90 degrees+half ∠B)

= half a+half b

Because ∠A+∠B equals 90.

So what? Half? Besides? Half? ∠B equals? 45

So what? ∠ECD equals? 45

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