ISI MStat 2016 Problem 1 | Area bounded by the curves | PSB Sample
This is a beautiful problem from ISI MStat 2016 Problem 1, PSB Sample, based on area bounded by the curves. We provide a detailed solution with the prerequisites mentioned explicitly.
Problem- ISI MStat 2016 Problem 1
In the diagram below, \(L(x)\) is a straight line that intersects the graph of a polynomial \(P(x)\) of degree 2 at the points \(A=(-1,0)\) and \(B=(5,12) .\) The area of the shaded region is 36 square units. Obtain the expression for \(P(x)\).

Prerequisites
- Area bounded by the curve
- Polynomials of degree 2
- Area of a triangle
Solution
Let \(P(x)=ax^2 +bx + c \) as given \(P(x)\) is of degree 2 .
Now from the figure we can see that \(L(x)\) intersect \(P(x)\) at points \(A=(-1,0)\) and \(B=(5,12) .\)
Hence we have \(P(-1)=0\) and \(P(5)=12\) , which gives ,
\( a-b+c=0 \) ---(1) and \( 25a+5b +c =12 \) ----(2)
Then ,

See from Fig-1 we can say that Area of the shaded region = (Area bounded by the curve P(x) and x-axis )- (Area of the triangle ABC) - (Area bounded by the curve P(x) , x=5 and x=L )
= \( \int^{L}_{-1} P(x)\,dx - \frac{1}{2} \times (5+1) \times 12 -\int^{5}_{L} P(x)\,dx \)
=\(\int^{L}_{-1} P(x)\,dx - \int^{5}_{L} P(x)\,dx \) -36
=\( \int^{5}_{-1} P(x)\,dx \) -36
Again it is given that area of the shaded region is 36 square units.
So, \( \int^{5}_{-1} P(x)\,dx \) -36 =36 \( \Rightarrow \) \( \int^{5}_{-1} P(x)\,dx \) =\( 2 \times 36 \)
\( \int^{5}_{-1} (ax^2+bx+c) \,dx = 2 \times 36 \) . After integration we get ,
\( 7a + 2b +c =12 \) ---(3)
Now we have three equations and three unknows
\( a-b+c=0 \)
\( 25a+5b +c =12 \)
\( 7a + 2b +c =12 \)
Solving this three equations by elimination and substitution we get ,
\( a=-1 , b=6 , c=7 \)
Therefore , the expression for \(P(x)\) is \( P(x)= -x^2+6x+7 \) .


![[asy] unitsize(10mm); defaultpen(linewidth(.8pt)+fontsize(10pt)); dotfactor=4; pair A=(0,0), B=(5,0), C=(1.8,2.4), D=(5-4sqrt(3)/3,0), E=(5-4sqrt(3)/3,sqrt(3)); pair[] ps={A,B,C,D,E}; draw(A--B--C--cycle); draw(E--D); draw(rightanglemark(E,D,B)); dot(ps); label("$A$",A,SW); label("$B$",B,SE); label("$C$",C,N); label("$D$",D,S); label("$E$",E,NE); label("$3$",midpoint(A--C),NW); label("$4$",midpoint(C--B),NE); label("$5$",midpoint(A--B),SW); [/asy]](https://latex.artofproblemsolving.com/9/5/3/953e7cbe2b2ee84c7c3ff7f6c642569bc2483c4e.png)






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