2條f.2升f.3數學(三角形)

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(1)

∠DAC = ∠ADC = 72°
∠ACD = 180° - 72° - 72° = 36°
∠BCD = 36°/2 = 18°
∠BDC = 180° - 72° = 118°
x = 180° - 18° - 118° = 44°

(2)

∠AEC = 180° - 78° = 112°
∠EAC = ∠ECA
∠EAC = (180°-112°)/2 = 78°/2 = 39°
∠CAD = ∠EAC = 39°
∠ACD = ∠ADC = x
x = (180°-39°)/2 = 141°/2 = 70.5°
∠ECA = 39°
∠ACD = 70.5°
y = 180° - 39° - 70.5° = 70.5°

1.
∠CAB=72° (BASE ∠ ISOS. △)
∠ACD+72°+72°=180° (∠ SUM OF △)
∠ACD=36°
∠BCD=36°/2
=18°
x+72°+18°+36°=180° (∠ SUM OF △)
x=54°


2.
設 a=∠EAC.
a=∠ACE (BASE ∠ ISOS. △)
a+a=78°(EXT. ∠ OF △)
a=39°
∠EAC=∠DAC(GIVEN)
∠DAC=39°
x=∠ACD (BASE ∠ ISOS. △)
2x+39°=180° (∠ SUM OF △)
x=70.5°

∠B+70.5°+39°+39°=180° (∠ SUM OF △)
∠B=31.5°
y+31.5°+78°=180° (∠ SUM OF △)
y=70.5°