If the area of the region {(x,y):∣x−5∣≤y≤4x} is A , then 3 A is equal to
Answer: 368
Solution
The boundaries y=∣x−5∣ and y=4x meet where 4x=∣x−5∣: for x<5 this gives x=1, and for x≥5 it gives x=25. The area is A=∫1254xdx−∫125∣x−5∣dx. The first integral is 38(253/2−1)=38(124)=3992. The region under ∣x−5∣ is two triangles of areas 21(4)(4)=8 and 21(20)(20)=200, together 208. Hence A=3992−208=3368, and 3A=368.