函数设y=sinX 证明Δy=2cos(X+1/2ΔX)sin1/2ΔX

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函数设y=sinX 证明Δy=2cos(X+1/2ΔX)sin1/2ΔX
函数
设y=sinX 证明Δy=2cos(X+1/2ΔX)sin1/2ΔX

函数设y=sinX 证明Δy=2cos(X+1/2ΔX)sin1/2ΔX
Δy=sin(X+ΔX)-sin X
=cos X sin(ΔX) + sin X cos ΔX - sin X
=2cos X cos(1/2ΔX) sin(1/2ΔX) + sin X (cos ΔX - 1)
=2cos X cos(1/2ΔX) sin(1/2ΔX) - 2 sin X sin(1/2ΔX)^2
=2 [cos X cos(1/2ΔX) - sin X sin(1/2ΔX)] sin(1/2ΔX)
=2cos(X+1/2ΔX)sin1/2ΔX

Δy=sin(X+ΔX)-sin X
Δy=cos X sin(ΔX) + sin X cos ΔX - sin X
Δy=2cos X cos(1/2ΔX) sin(1/2ΔX) + sin X (cos ΔX - 1)
Δy=2cos X cos(1/2ΔX) sin(1/2ΔX) - 2 sin X sin(1/2ΔX)^2
Δy=2 [cos X cos(1/2ΔX) - sin X sin(1/2ΔX)] sin(1/2ΔX)
Δy=2cos(X+1/2ΔX)sinΔX/2

Δy=sin(X+ΔX)-sin X