cosπ/2加t等於多少

cos(π/2+t)=-sint。三角函數口訣：奇變偶不變，符號看象限。π/2爲π/2的奇數倍，因此cos變爲sinπ/2+t屬於第二象限，cos的值爲負數。cos0°=1、cos15°=（√6+√2）/4、cos30°=√3/2

cos45°=√2/2、cos60°=1/2、cos75°=sin15°、cos90°=0

餘弦定理的公式

a b c爲三角形3邊 A B C爲3邊所對角

cosA=(b^2+c^2-a^2)/2bc

cosB=(a^2+c^2-b^2)/2ac

cosC=(a^2+b^2-c^2)/2ab

c^2=a^2+b^2-2ab*cosC

cos(a-b)=cosacosb+sinasinb

cos3a

=cos(2a+a)

=cos2acosa-sin2asina

=(2cos^2a-1)cosa-2(1-cos^2a)cosa

=4cos^3a-3cosa

三角函數cos公式

cos(-a) = cos(a) 

sin(π/2 - a) = cos(a)  

cos(π/2 - a) = sin(a)  

sin(π/2 + a) = cos(a)  

cos(π/2 + a) = - sin(a)

cos(π - a) = - cos(a)

cos(π + a) = - cos(a)

sin(a + b) = sin(a)cos(b) + cos(α)sin(b)  

cos(a + b) = cos(a)cos(b) - sin(a)sin(b)

sin(a - b) = sin(a)cos(b) - cos(a)sin(b) 

cos(a - b) = cos(a)cos(b) + sin(a)sin(b)

sin(a) + sin(b) = 2sin[(a + b)/2]cos[(a - b)/2]    

sin(a) - sin(b) = 2sin[(a - b)/2]cos[(a + b)/2]