利用和(差)角公式求下列各三角函数值

sin(-7π/12)
= -sin(7π/12)
= -sin﹙π/3+π/4﹚
= -(sinπ/3cosπ/4+cosπ/3sinπ/4)
= -(√3/2 * √2/2 + 1/2 * √2/2)
= -(√6+√2)/4
cos(-61π/12）
= cos(61π/12)
= cos(5π+π/12)
= cos(π+π/12)
= -cos(π/12)
= -cos(π/3-π/4)
= -(cosπ/3cosπ/4+sinπ/3sinπ/4)
= -(1/2 * √2/2 + √3/2 * √2/2)
= -(√2+√6)/4
tan(35π/12)
= tan(3π-π/12)
= tan(π-π/12)
= -tan(π/12)
= -tan(π/3-π/4)
= -(tanπ/3-tanπ/4) / (1+tanπ/3tanπ/4)
= -(√3-1) / (1+√3)
= -(√3-1)^2 /{ (√3+1)(√3-1)}
= -(4-2√3)/(3-1)
= √3 - 2