Let f ( x ) = a x 4 + b x 2 + c and g ( x ) = cos ( x ) , where f ( 0 ) = g ( 0 ) , f ( 2 π ) = g ( 2 π ) and ∫ 0 2 π f ( x ) d x = ∫ 0 2 π g ( x ) d x .
If the difference D of the areas of f ( x ) and g ( x ) on the interval [ 0 , 4 π ] can be expressed as D = β α ∗ β α β π + λ − β γ β , where α , β , λ and γ are coprime positive integers, find α + β + λ + γ .
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Problem Loading...
Note Loading...
Set Loading...
f ( 0 ) = g ( 0 ) = 1 ⟹ c = 1 and f ( 2 π ) = g ( 2 π ) = 0 ⟹ π 4 a + 4 π 2 b = − 1 6 and ∫ 0 2 π g ( x ) d x = ∫ 0 2 π cos ( x ) d x = 1 ⟹ 1 = ∫ 0 2 π ( a x 4 + b x 2 + 1 ) d x = 5 a x 5 + 3 b x 3 + x ∣ 0 2 π = 5 ∗ 2 5 π 5 a + 3 ∗ 2 3 π 3 b + 2 π ⟹
3 π 5 a + 2 0 π 3 b = 4 8 0 − 2 4 0 π
π 4 a + 4 π 2 b = − 1 6
Solving the system above we obtain: a = π 5 8 0 ( π − 3 ) and b = π 3 1 2 ( 5 − 2 π ) ⟹
∫ 0 4 π f ( x ) d x = ∫ 0 4 π ( π 5 8 0 ( π − 3 ) x 4 + π 3 1 2 ( 5 − 2 π ) x 2 + 1 ) d x = π 5 1 6 ( π − 3 ) x 5 + π 3 4 ( 5 − 2 π ) x 3 + x ∣ 0 4 π =
4 3 π − 3 + 4 2 5 − 2 π + 4 π = 6 4 π − 3 + 2 0 − 8 π + 1 6 π = 6 4 9 π + 1 7
and,
∫ 0 4 π g ( x ) d x = ∫ 0 4 π cos ( x ) d x = 2 1 ⟹
D = ∫ 0 4 π f ( x ) − g ( x ) d x = 6 4 9 π + 1 7 − 2 2 = 6 4 9 π + 1 7 − 3 2 2 = 2 2 ∗ 3 3 2 π + 1 7 − 2 5 2 = β α ∗ β α β π + λ − β γ β ⟹ α + β + λ + γ = 2 7 .
Note: I didn't show the graph in the problem, since the graph suggest that the areas are equal on [ 0 , 4 π ] which is not the case. As can be seen from the solution the difference is small ( 6 4 9 π + 1 7 − 3 2 2 ≈ 0 . 0 0 0 3 0 4 6 8 5 7 ).