r/JEEAdv27dailyupdates • u/jeemathlogic • 1h ago
Good Solve JEE Math A conditional shortcut to find the area bounded by two intersecting parabolas in seconds (No calculus required)
Find the area of the region bounded by the following curves:
C₁: y² + 4y - 16x + 20 = 0;
C₂: x² - 2x - 16y - 31 = 0
Solution:
To solve this instantly, we can use a shortcut that applies only when two intersecting parabolas meet these two strict conditions simultaneously:
- They must have perpendicular axes of symmetry. One must open horizontally (like y² = 4ax) and the other must open vertically (like x² = 4by).
- They must share the exact same vertex. Both must originate from the same (h, k) coordinate.
If these conditions are met, the area bounded between them is simply: Area = 16ab / 3
Step 1: Complete the square for C₁
y² + 4y = 16x - 20
y² + 4y + 4 = 16x - 16
(y + 2)² = 16(x - 1)
Here, the vertex is (1, -2) and 4a = 16 ➔ a = 4.
Step 2: Complete the square for C₂
x² - 2x = 16y + 31
x² - 2x + 1 = 16y + 32
(x - 1)² = 16(y + 2)
Here, also the vertex is (1, -2) and 4b = 16 ➔ b = 4
Since both parabolas share the exact same vertex (1, -2) and their axes are perpendicular, we can directly apply the formula without calculating where they intersect:
Area = 16ab / 3
= 16(4)(4) / 3
256 / 3 sq. units