JAMB 2014 Mathematics · Question 18

Evaluate ∫(2x+3)12δx\int (2x + 3)^{\frac{1}{2}} \delta x

Options

  1. A.112(2x+3)6+k\frac{1}{12} (2x + 3)^6 + k
  2. B.13(2x+3)12+k\frac{1}{3} (2x + 3)^{\frac{1}{2}} + k
  3. C.13(2x+3)32+k\frac{1}{3} (2x + 3)^{\frac{3}{2}} + k
  4. D.112(2x+3)34+k\frac{1}{12} (2x + 3)^{\frac{3}{4}} + k

Answer: C. 13(2x+3)32+k\frac{1}{3} (2x + 3)^{\frac{3}{2}} + k

Explanation written with AI and checked against the answer key

To solve the integral ∫(2x+3)12 dx\int (2x + 3)^{\frac{1}{2}} \, dx, we use the method of Integration by Substitution (or the reverse chain rule).

1. Analyze the Concept

When integrating a function of the form (ax+b)n(ax + b)^n, the rule is:

∫(ax+b)n dx=(ax+b)n+1a(n+1)+k\int (ax + b)^n \, dx = \frac{(ax + b)^{n+1}}{a(n+1)} + k

Where:

  • aa is the coefficient of xx (in this case, 22).
  • nn is the power (in this case, 12\frac{1}{2}).
  • kk is the constant of integration.
2. Step-by-Step Solution

Step 1: Identify the components.

  • n=12n = \frac{1}{2}
  • a=2a = 2

Step 2: Apply the integration formula. Increase the power by 1:

n+1=12+1=32n + 1 = \frac{1}{2} + 1 = \frac{3}{2}

Divide by the new power and multiply by the derivative of the inner function (which is 22):

∫(2x+3)12 dx=(2x+3)322×(32)+k\int (2x + 3)^{\frac{1}{2}} \, dx = \frac{(2x + 3)^{\frac{3}{2}}}{2 \times (\frac{3}{2})} + k

Step 3: Simplify the expression. The 22 in the denominator and the 22 in the denominator of the fraction 32\frac{3}{2} cancel out:

2×32=32 \times \frac{3}{2} = 3

So, the expression becomes:

(2x+3)323+k\frac{(2x + 3)^{\frac{3}{2}}}{3} + k

Which can be written as:

13(2x+3)32+k\frac{1}{3} (2x + 3)^{\frac{3}{2}} + k
3. Conclusion

Comparing our result to the given options, we find that it matches option C.

Exam Tip: Whenever you see a linear expression inside a power (like 2x+32x+3), always remember to divide by the coefficient of xx. Many students forget to divide by the 22, which leads to common errors. Always check if your answer differentiates back to the original function to verify your work!

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