Ok guys here we go. this isI jsut a suggested question from our teacher in prep for our final. it a integral one. trig substion the prob I am having is near the end.. all the substition works great was down to the last bit and for some reason I must be doing the back substition wrong. so here it is. orginal question Integral of Sqr(1-4x^2) dx I do the substion of x=1/2 sin @ (@= theta) and do all that stuff. after I integrat i have 1/4 @ + 1/8 sin2@ so i do the back sub for just @ i get arcsine 2x which is correct but some reason i don;t get the back end right... some how 1/8 sin2@ = 1/2*x*sqr(1-4x^2) so if you could fight out the why i am going wrong and screwing up could you tell me.