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2Cos10 sin70

sin20=sin(30-10) 所以 (2cos10°—sin20°)/sin70° =(2cos10°—sin(30-10))/sin70° =(2cos10-(sin30cos10-cos30sin10))/sin70 =(2cos10-1/2cos10+根号3/2sin10)/sin70 =(3/2sin10+根号3/2sin10)/sin70 =根号3(根号3/2cos10+1/2sin10)/sin70 =根...

即在花简计算用到 sin20=sin(30-10) 所以 (2cos10°—sin20°)/sin70° =(2cos10°—sin(30-10))/sin70° =(2cos10-(sin30cos10-cos30sin10))/sin70 =(2cos10-1/2cos10+根号3/2sin10)/sin70 =(3/2sin10+根号3/2sin10)/sin70 =根号3(根号3/2cos10+1/2...

sin20=sin(30-10) 所以 (2cos10°—sin20°)/sin70° =(2cos10°—sin(30-10))/sin70° =(2cos10-(sin30cos10-cos30sin10))/sin70 =(2cos10-1/2cos10+根号3/2sin10)/sin70 =(3/2sin10+根号3/2sin10)/sin70 =根号3(根号3/2cos10+1/2sin10)/sin70 =根...

原式= (2cos10-sin20)/sin70 =(2cos10-sin20)/cos20 =[cos10+(cos10-cos70)]/cos20 =[cos10+2sin40*sin30]/cos20 =[cos10+2*1/2*sin40]/cos20 =[cos10+cos50]/cos20 =2cos30*cos20/cos20 =2cos30 = 根号3

sin20=sin(30-10) 所以 (2cos10°—sin20°)/sin70° =(2cos10°—sin(30-10))/sin70° =(2cos10-(sin30cos10-cos30sin10))/sin70 =(2cos10-1/2cos10+根号3/2sin10)/sin70 =(3/2sin10+根号3/2sin10)/sin70 =根号3(根号3/2cos10+1/2sin10)/sin70 =根...

原式= (2cos10-sin20)/sin70 =(2cos10-sin20)/cos20 =[cos10+(cos10-cos70)]/cos20 =[cos10+2sin40*sin30]/cos20 =[cos10+2*1/2*sin40]/cos20 =[cos10+cos50]/cos20 =2cos30*cos20/cos20 =2cos30 = 根号3

:(2cos10º-sin20º)/sin70° =[2cos10º-sin(30º-10º)]/sin70º =[2cos10º-(sin30ºcos10º-cos30ºsin10º)]/sin70º =(2cos10º-(1/2)cos10º+(√3/2)sin10º)/sin70º =...

原式 =2sin20+[2(cos5)^2-1]+sin10sin20/cos20+1 =2sin20+cos10+sin10sin20/cos20+1 =2sin20+2cos10+(sin10sin20-cos10cos20)/cos20+1 =2sin20+2cos10-cos(20+10)/cos20+1 =2sin20cos20/cos20-cos30/cos20+2cos10+1 =sin40/cos20-sin60/cos20+2c...

因为30度的sin和cos 都是常数 式子里都是20度 化成常数和统一的度数好计算

乘上一个2cos10 再除一个2cos10 就变成sin20*sin30 sin50sin70 sin7-转化为为cos20,就变成1/2sin40 sin30 sin50 40与50一转化 结果就出来了

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