问题标题:
【求等差、等比数列的前n项和公式,以及三角涵数常用的变换公式另外再问问,secx=1/cosx,cscx=1/sinx是否正确?太久不用记不清了,】
问题描述:

求等差、等比数列的前n项和公式,以及三角涵数常用的变换公式

另外再问问,secx=1/cosx,cscx=1/sinx是否正确?太久不用记不清了,

白经天回答:
  等差:Sn=n(a1+an)/2,Sn=na1+n(n-1)d/2   等比:Sn=a1(1-q^n)/(1-q),Sn=(a1-anq)/(1-q)注意q不=1   三角:   1.基本关系:(sinx)^2+(cosx)^2=1,tanx=sinx/cosx,secx=1/cosx,cscx=1/sinx,(secx)^2=1+(tanx)^2,(cscx)^2=1+(cotx)^2   2.二倍交公式:sin2x=2sinx*cosx,变形:sinx*cosx=sin2x/2   1+sinx=(sin(x/2))^2+(cos(x/2))^2+2sin(x/2)cos(x/2)=[sin(x/2)+sin(x/2)]^2   1-sinx=(sin(x/2))^2+(cos(x/2))^2-2sin(x/2)cos(x/2)=[sin(x/2)-sin(x/2)]^2   cos2x=(cosx)^2-(sinx)^2   =2(cosx)^2-1,变形:(cosx)^2=(1+cos2x)/2   =1-2(sinx)^2,变形:(sinx)^2=(1-cos2x)/2   tan2x=2tanx/(1-(tanx)^2)   3.两角和与差公式:sin(x+y)=sinxcosy+sinycosx,sin(x-y)=sinxcosy-sinycosx   cos(x+y)=cosxcosy-sinxsiny,cos(x-y)=cosxcosy+sinxsiny   tan(x+y)=(tanx+tany)/(1-tanxtany),变形:tanx+tany=tan(x+y)(1-tanxtany)   tan(x-y)=(tanx-tany)/(1+tanxtany),变形:tanx-tany=tan(x-y)(1+tanxtany)   4.万能公式:sin2x=2tanx/(1+(tanx)^2),   cos2x=(1-(tanx)^2)/(1+(tanx)^2)
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