∫x/(4+9*x^2)dx
=(1/2)∫1/(4+9*x^2)dx^2
=(1/18)∫1/(4+9*x^2)d(4+9x^2)
=(1/18)*ln(4+9x^2) (0到2)
=（1/18）*(ln40-ln4)
=(1/18)ln10
∫x/(4+9*x^2)dx
=(1/18)*ln(4+9x^2)+C