Question 1192775
You didn't mention how to get there so I'll be straightforward.
{{{y=x+cos(x)}}}
{{{dy/dx=1-sin(x)}}}
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Approximate change? Why not exact change in y?
{{{y[2]=pi/5+cos(pi/5)}}}
{{{y[1]=pi/6+cos(pi/6)}}}
So the change in y would be,
{{{y[2]-y[1]=pi/5+cos(pi/5)-(pi/6+cos(pi/6))}}}
{{{y[2]-y[1]=pi/5-pi/6+cos(pi/5)-cos(pi/6)}}}
Approximate from there.