Find the area enclosed by the curve y = –x2 and the straight lilne x + y + 2 = 0.
y = -x2⇒ x2 = -y
x2 = -y parabola is not defined for positive values of y hence parabola will be below X-axis opening downwards and passing through (0, 0)
x + y + 2 = 0 is a straight line
To find point of intersection of parabola and straight line solve the parabola equation and the straight line equation simultaneously
Put y = -(x + 2) in x2 = -y
⇒ x2 = -(-(x + 2))
⇒ x2 = x + 2
⇒ x2 – x – 2 = 0
⇒ x2 – 2x + x – 2 = 0
⇒ x(x – 2) + 1(x – 2) = 0
⇒ (x + 1)(x – 2) = 0
⇒ x = -1 and x = 2
Put x = -1 in x2 = -y
⇒ (-1)2 = -y
⇒ y = -1
Put x = 2 in x2 = -y
⇒ 22 = -y
⇒ y = -4
Hence the parabola and line intersects at (-1, -1) and (2, -4)
Plot the parabola and straight line and the bounded area is as shown
⇒ area enclosed by line and parabola = area under line – area under parabola …(i)
Let us find area under the straight line
x + y + 2 = 0
⇒ y = -(x + 2)
Integrate from -1 to 2
Now let us find area under parabola
x2 = -y
⇒ y = -x2
Integrate from -1 to 2
Using (i)
⇒ area enclosed by line and parabola = = unit2
We are getting negative sign because the area is below X-axis as seen in figure
Hence area enclosed is unit2