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Area Under the Curves question
2017 · Shift 0 · Q38
JEE MainMathematicsArea Under the CurvesMCQ+4 / −1
The area (in sq. units) of the region {(x,y):x≥0,x+y≤3,x2≤4yandy≤1+x} is
A
23
B
37
C
25
D
1259
View written solutionFree
Correct answer: C
Interpret the region
We need the area of the set
{(x,y):x≥0,x+y≤3,x2≤4y,y≤1+x}.
Rewrite each inequality in a more useful form:
x≥0
x+y≤3⟹y≤3−x
x2≤4y⟹y≥4x2
y≤1+x
So for each x≥0, the region lies abovey=4x2
and below bothy=3−xandy=1+x.
Hence upper boundary is
min(3−x,1+x).
So area is determined by comparing the two upper curves.
Find intersection of the two upper curves
Solve
3−x=1+x.
Then
2−x=x.
Let t=x, so x=t2 with t≥0.
Then
2−t2=tt2+t−2=0(t+2)(t−1)=0.
Since t≥0, we get t=1, hence
At x=1, both give y=2.
Now check which is smaller:
At x=0: 3−x=3, 1+x=1, so 1+x is smaller.
For x>1, 3−x becomes smaller.
Thus,
for 0≤x≤1, upper curve is y=1+x,
for x≥1, upper curve is y=3−x.
Find where the lower curve meets the relevant upper curves
The lower curve is
y=4x2.
(i) Intersection with y=1+x
Solve
4x2=1+x.
Let t=x, so x=t2.
Then
4t4=1+tt4=4t+4.
Check t=2:
t4=4,4t+4>4,
so not equal. Instead test x=4:
so intersection is not in the interval near [0,1]. Since on 0≤x≤1, we have
4x2≤41<1+x,
there is no issue there; region exists throughout [0,1].
(ii) Intersection with y=3−x
Solve
4x2=3−x.
Multiply by 4:
x2=12−4xx2+4x−12=0(x+6)(x−2)=0.
Since x≥0, we get
Thus for x>2, the lower curve exceeds 3−x, so no region exists.