1.

Find ∫7 cos⁡mx dx.(a) \(\frac{7 \,sin⁡mx}{x}+C\)(b) \(\frac{7 \,sin⁡mx}{m}+C\)(c) \(\frac{sin⁡mx}{x}+C\)(d) \(\frac{sin⁡x}{m}+C\)I got this question in an interview for job.This is a very interesting question from Methods of Integration-1 in chapter Integrals of Mathematics – Class 12

Answer»

The correct ANSWER is (b) \(\FRAC{7 \,sin⁡mx}{m}+C\)

Easy explanation: Using Integration by Substitution, LET xm=t

Differentiating w.r.t x, we get

mdx=dt

∴\(\int 7 \,cos⁡mx \,dx=\int \frac{(7 cos⁡t)}{m} dt\)

=\(\frac{7}{m} \int cos⁡t \,dt=\frac{7}{m} (sin⁡t)+C\)

REPLACING t with mx again we get,

\(\int 7 \,cos⁡mx \,dx=\frac{7 \,sin⁡mx}{m}+C\)



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