求值1/2sin138°(根号3sin228°/cos132°+1/sin270°)+cos252°
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求值1/2sin138°(根号3sin228°/cos132°+1/sin270°)+cos252°
1/2sin138°(√3sin228°/cos132°+1/sin270°)+cos252°
=1/2sin(180°-42°)[√3sin(180°+48°)/cos(180°-48°)+1/sin(360°-90°)]+cos(180°+72°)
=1/2sin42°[√3sin48°/cos48°-1/sin90°]-cos72°
=1/2cos48°[√3sin48°/cos48°-1]-cos72°
=(√3/2)sin48°-(1/2)cos48°-cos72°
=cos30°sin48°-sin30°cos48°-cos72°
=sin(48°-30°)-cos72°
=sin18°-cos(90°-18°)
=sin18°-sin18°
=0
=1/2sin(180°-42°)[√3sin(180°+48°)/cos(180°-48°)+1/sin(360°-90°)]+cos(180°+72°)
=1/2sin42°[√3sin48°/cos48°-1/sin90°]-cos72°
=1/2cos48°[√3sin48°/cos48°-1]-cos72°
=(√3/2)sin48°-(1/2)cos48°-cos72°
=cos30°sin48°-sin30°cos48°-cos72°
=sin(48°-30°)-cos72°
=sin18°-cos(90°-18°)
=sin18°-sin18°
=0
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