为什么8sinπ/9与sin8π/9能约然后余8
\uff08sin7\u00b0+cos15\u00b0*sin8\u00b0\uff09/(cos7-sin15\u00b0*sin8\u00b0)(sin7+cos15sin8)/(cos7-sin15sin8)
=[sin7+cos(7+8)sin8]/[cos7-sin(7+8)sin8]
=[sin7+(cos7cos8-sin7sin8)sin8]/[cos7-(sin7cos8+sin8cos7)sin8]
=(sin7+cos7cos8sin8-sin7sin²8)/(cos7-sin7sin8cos8-sin²8cos7)
=(sin7-sin7sin²8+cos7cos8sin8)/(cos7-sin²8cos7-sin7sin8cos8)
=[sin7(1-sin²8)+cos7cos8sin8]/[cos7(1-sin²8)-sin7sin8cos8]
=(sin7cos²8+cos7cos8sin8)/(cos7cos²8-sin7sin8cos8)
=(sin7cos8+cos7sin8)/(cos7cos8-sin7sin8) {\u6bcf\u9879\u9664\u4ee5cos8}
=sin(7+8)/cos(7+8)
=sin15/cos15
=tan15
=tan(45-30)
=[tan45-tan30]/[1-tan45*tan30]
=[1-\u6839\u53f73/3]/(1-\u6839\u53f73/3)
=2-\u6839\u53f7(3)
sin8a/8sina
=2sin4a*cos4a/8sina
=4sin2a*cos2a*cos4a/8sina
=cosa*cos2a*cos4a
（8sinπ/9）/（sin8π/9）=-8
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