一道高中三角函数数学题，好的加悬赏到100
\u4e00\u9053\u9ad8\u4e2d\u6570\u5b66\u9898\uff08\u4e09\u89d2\u51fd\u6570\uff09\u4e24\u4e2a\u89e3\u662f\u4ecesinxcosx=-12/25\u63a8\u51fa\u6765\u7684\uff0c\u663e\u7136\u6709sinx=3/5,cosx=-4/5\u6216sinx=4/5,cosx=-3/5\u5747\u6ee1\u8db3\u6761\u4ef6\uff0c\u8fd9\u6bd4\u539f\u6761\u4ef6sinx+cosx=-1/5\u653e\u5bbd\u4e86\uff0c\u56e0\u6b64\u4e24\u4e2a\u89e3\u8fd8\u8981\u4ee3\u5165\u539f\u6761\u4ef6\u770b\u662f\u5426\u6ee1\u8db3\u3002
\u5e94\u8be5\u7efc\u5408\u5229\u7528\u5b83\u4eec\uff0c\u6bd4\u5982sinx\uff0ccosx\u662f\u65b9\u7a0by^2-y/5-12/25=0\u7684\u4e24\u6839
\u5f97y1=4/5,y2=-3/5
\u663e\u7136sinx=4/5,cosx=-3/5
\u6240\u4ee5tanx=-4/3
\u5982\u56fe\u89d2\u5927\u6982\u662f37.5\u5ea6\uff0c\u8fd9\u662f\u4e00\u4e2a345\u7684\u76f4\u89d2\u4e09\u89d2\u5f62\u3002
∴ sina<0,cos>0
(1)化简f(a)=sina*cosa*（-sina)/(sina*(-tana))
=-sina*cosa*cosa/sina=-(cosa)^2
(2)cos【α-（3π/2）】=cos（3π/2-a）=-sina=1/5
sina=-1/5
f(a)=-(cosa)^2=(sina)^2-1=-24/25
(3)a=-1860°=-360°*5-60°=-2π*5-π/3
cos(-2π*5-π/3)=cos(2π*5+π/3)=cosπ/3
f(a)=-(cosa)^2=-(cosπ/3)^2=-1/4
注：sin(π-a) = sina
cos（2π-α）= cosα
cos（ 3π/2-α）= -sinα(特别注意是负的sin，不是+sin)
sin（-α）= -sinα
cos（-α）= cosα
1、原式=(-sinacosasina)/(-sinatana)=cos²a
2、cos【α-（3π/2）】= -sina=1/5,sina= -1/5
f(a)=cos²a=1-sin²a=24/25
3、α=-1860°= -(10π+π/3)
f(a)=cos²(10π+π/3)=1/4
f(a)=(sina*cosa*sina)/(sina*tana)=cosa2;
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