Calculus
posted by Luke
find d^2y/dx^2 (second derivative) for y=cos^2 4x

bobpursley
y=cos^2 4x
y'=2cos 4x * sin 4x*4= 8cos4x*sin4x
= 4sin(8x)
y"= 4cos(8x)(8)=32 cos(8x)
y"'=+32*8 sin(8x)
y""=32*64*cos(8x)
check my work 
Luke
how did you get 4sin8x from 8cos4x*sin4x?
The answer is 32(cos^2 4xsin^2 4x) but not sure how to get there 
What Zit Tooya
y'=8sin(4x)cos(4x)
y"=8[sin(4x)(sin(4x))*4+cos(4x)(cos(4x))*4]
y"=8[4sin^2(4x)+4cos^2(4x)]
y"=32[cos^2(4x)sin^2(4x)]
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