cos(a-π/6)+sina=4√3/5
cosacosπ/6+sinasinπ/6+sina=4√3/5
√3/2*cosa+3/2*sina=4√3/5
1/2*cosa+√3/2*sina=4/5
∴sin(a+π/6)=4/5
∴sin(a+7π/6)=sin(π+a+π/6)
=-sin(a+π/6)
=-4/5
cos(a-π/6)+sina=4√3/5
=>sin(a+π/6)=4/5
∴cos(a+π/6)=±3/5
sin(a-7π/6+π/6-π/6)
=sin(a+π/6-4π/3)
=sin(a+π/6)cos(4π/3)-cos(a+π/6)sin(4π/3)
=-sin(a+π/6)cos(π/3)+cos(a+π/6)sin(π/3)
=-4/10±3√3/10
=(-4±3√3)/10
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