x+y=tan(y), 1+y'=y'sec2(y), y'(sec2(y)-1)=1,
y'tan2(y)=1, y'=cot2(y), y'2=cot4(y);
y"tan2(y)+2y'2tan(y)sec2(y)=0,
y"tan2(y)=-2y'2tan(y)sec2(y),
y"=-2y'2cot(y)(1+tan2(y))=-2y'2(cot(y)+tan(y))=-2(cot5(y)+cot3(y)),
which can be written y"=-(2cot3(y)+2cot5(y)) QED