You are here:

Advertisement

Sir,

I wish to draw your attention to your reply dt. Nov 3 to my Q 2 regarding integral of dx/(4-5 sin x). I reproduce an extract below-

"(2)...attempt a completing the square to make (2t^2-5t+2) be equal to 2(t-5/4)^2-9/8,then try a w-substitution with w = t-5/4 and you will be having integral of dw/(2w^2 -9/8). Now bring out a factor of (-2) from the denominator, and you have a similar integral...."

I tried and repeatedly got 1/3*log[2(t-2)/2t-1)] instead of 1/3*log[(t-2)/2t-1)] that I get from Partial Fractions and is given as correct answer. (t=tan x/2)

s=1/2*Int dw/(w^2- 9/16=1/2*1/2*4/3*log[(w- 3/4)/(w+ 3/4)]

=1/3*log[2(t-2)/(2t-1)]

I assume that this is =1/3*log[(t-2)/2t-1)]+log 2, and the second term is accounted for by constant of integration.

Kindly advise if I am correct or whether I am making a mistake somewhere.

Yes, both answers 1/3*log[2(t-2)/2t-1)] and 1/3*log[(t-2)/2t-1)] are correct. To know whether F(x) is an integral of f(x), simply check if F'(x) is f(x). Since both answers differ only by a constant (and the derivative of a constant is zero), both answers are correct.

Sometimes, it is not so easy to see that TWO different answers differ only by a constant, for example, when integrating sec^2(3x) tan^3(3x) dx,

one could do u = tan (3x) (du = 3sec^2(3x) dx ) or u = sec(3x) (then du=3 sec(3x) tan(3x) dx ) and get two very different answer. (Try it, since you know many techniques of integration, this may further help you in sharpening your Math skills. ) Unless one is extremely good with trigonometry, one may not even know what constant the two different answers differ by.

Calculus

Answers by Expert:

I can answer all calculus question. I am a Math Lecturer and I teach Math in a College, usually Calculus 1, Calculus 2 and Calculus 3 and Linear Algebra.

I have been teaching in this college for more than 12 years. Prior to that, I taught for three years as Visiting Assistant Professor.**Education/Credentials**

I got my doctorate from Univ. of Rochester in Algebraic Toppology. I got a MA from Univ of Rochester, an MA from York Univ. in Toronto and almost did my M.Sc in the National Univ. of Singapore.