If the area of the region is , then is equal to:
- A
- B
- C
- D
If the area of the region is , then is equal to:
Correct answer:A
Standard Method
Given: The region is defined by .
Find: The value of , where is the area of the region.
First identify which line gives the upper boundary. The two lines intersect at
so
and hence
Therefore, for the minimum is , and for the minimum is .
Now find where the parabola meets each line in the relevant interval.
With ,
so the roots are and . Since this part uses , the valid point is .
With ,
so the roots are and . Since this part uses , the valid point is .
Hence the area must be split into two parts:
where
and
For the first part,
Now,
and
Therefore,
For the second part,
Now,
and
Therefore,
So the total area is
Hence,
Therefore, the correct option is A.
The first solution shown in the source contains an arithmetic inconsistency; the second solution correctly gives .
Conclusion: The value of is .
Piecewise Upper Boundary Interpretation
Given: lies above the parabola and below the function .
Find: The required area expression and then compute .
Because the upper boundary is a minimum of two lines, we must use the lower of the two lines at each value of . Their intersection occurs at , so the upper boundary changes there. This is why the area is not found using a single integral over the whole interval.
The admissible region starts where the parabola first meets the active upper boundary on the left, namely at , and ends where it meets the active upper boundary on the right, namely at . Thus the region exists over
but with a change of upper curve at
Therefore,
Evaluating gives
Hence,
So the correct option is A.
Using a single upper boundary for the entire interval is incorrect because changes at . First compare the two lines, find where they intersect, and then split the integral at that point.
Taking both intersection points from each quadratic without checking the relevant interval is wrong. For , only the root with is valid, and for , only the root with is valid.
Subtracting in the wrong order gives a negative area. The integrand must always be upper curve minus lower curve, namely the active line minus .
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