>>392
訂正
>>360
√(1+4x^2)dx=√(1+(tanθ)^2)/(2(cosθ)^2)*dθ
=√((cosθ)^2)/(2(cosθ)^2))*dθ
=(1/2)dθcosθ/(cosθ)^2
=(1/2)dθ/cosθ