Answer:
We need to find dz/dt given:
z = cos(x + 8y), x = 7t^5, y = 5/t
Using the chain rule, we can find dz/dt by taking the derivative of z with respect to x and y, and then multiplying by the derivatives of x and y with respect to t:
dz/dt = dz/dx * dx/dt + dz/dy * dy/dt
First, let's find dz/dx and dz/dy:
dz/dx = -sin(x + 8y)
dz/dy = -8sin(x + 8y)
Now, let's find dx/dt and dy/dt:
dx/dt = 35t^4
dy/dt = -5/t^2
Substituting these values, we get:
dz/dt = (-sin(x + 8y)) * (35t^4) + (-8sin(x + 8y)) * (-5/t^2)
Simplifying this expression, we get:
dz/dt = -35t^4sin(x + 8y) + 40sin(x + 8y)/t^2
Substituting x and y, we get:
dz/dt = -35t^4sin(7t^5 + 40/t) + 40sin(7t^5 + 40/t)/t^2
Therefore, dz/dt is given by -35t^4sin(7t^5 + 40/t) + 40sin(7t^5 + 40/t)/t^2.