两角和与差的正弦，余弦，正切
cos165=cos(180-15)=-cos15=-cos（45-30） =-（cos45*cos30 sin45*sin30) =-√ 6/4-√ 2/4 =-(√ 6 √ 2)/4 cos12分之17派 =cos(180 75) =-cos75 =-cos(30 45) =--(cos30cos45-sin30sin45) =-(√6/4-√2/4) =(√2-√6)/4 cos100度cos20度减sin100度sin20度 =cos100*cos20-sin100*sin20 =cos(100 20) =cos120 =-cos60 =-1/2...全部
cos165=cos(180-15)=-cos15=-cos（45-30） =-（cos45*cos30 sin45*sin30) =-√ 6/4-√ 2/4 =-(√ 6 √ 2)/4 cos12分之17派 =cos(180 75) =-cos75 =-cos(30 45) =--(cos30cos45-sin30sin45) =-(√6/4-√2/4) =(√2-√6)/4 cos100度cos20度减sin100度sin20度 =cos100*cos20-sin100*sin20 =cos(100 20) =cos120 =-cos60 =-1/2 sin[-12分之7派]， =-sin105 =-sin(45 60) =-sin45cos60-cos45sin60 =-√2/4-√6/4 =-(√2 √6)/4 sin12分17派 =sin(180 75) =-sin(30 45) =-sin30cos45-cos30sin45 =-√2/4-√6/4 =-(√2 √6)/4 sin75度减sin15度 =sin75-sin15 =sin(90-15)-sin15 =cos15-sin15 =√2(cos15*cos45-sin15*sin45) =√2cos(15 45) =√2*1/2 =√2/2 cos75度减cos15度 =cos75-cos15 =cos(90-15)-cos15 =sin15-cos15 =√2(sin15*cos45-cos15*sin45) =√2*sin(-30) =-√2*1/2 =-√2/2。
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