已知cos(a-π/6)+sina=(4*根号3)/5,则sin(a-7π/6)的值是
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已知cos(a-π/6)+sina=(4*根号3)/5,则sin(a-7π/6)的值是
cos(a-π/6)+sina=cosacosπ/6+sinasinπ/6+sina=cosa*(根号3)/2+sina*1/2+sina
=cosa*(根号3)/2+sina*3/2=根号3*(cosa*1/2+sina*(根号3)/2=根号3*sin(π/6+a)
=(4*根号3)/5
所以sin(π/6+a)=4/5,
cos(π/6+a)=±1/5
则sin(a-7π/6)=-sin(a-π/6)=-sin[(a+π/6)-π/3]
=cos(a+π/6)sinπ/3-sin(a+π/6)cosπ/3
=±1/5*(根号3)/2-4/5*1/5
=cosa*(根号3)/2+sina*3/2=根号3*(cosa*1/2+sina*(根号3)/2=根号3*sin(π/6+a)
=(4*根号3)/5
所以sin(π/6+a)=4/5,
cos(π/6+a)=±1/5
则sin(a-7π/6)=-sin(a-π/6)=-sin[(a+π/6)-π/3]
=cos(a+π/6)sinπ/3-sin(a+π/6)cosπ/3
=±1/5*(根号3)/2-4/5*1/5
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