cos^2x + sin^2x = 1
2sinvcosv
cos^2v - sin^2v
sinxcosy-cosxsiny
sinxcosy+cosxsiny
cosxcosy+sinxsiny
cosxcosy-sinxsiny
(1-cosx)/2
(1+cosx)/2
a^2=b^2+c^2-2bccosA
(x-h)^2+(y-k)^2=r^2
ax^2+bx+c
(x^2/a^2)-(y^2/b^2)=1
(x^2/a^2)+(y^2/b^2)=1
n!/(k!(n-k))
n!/(n-k)!
a1 + (n-1)d
n*((a1+an)/2)
a1×k^(n-1)
a1×((r^n-1)/(r-1))