U² - 16 u + 64 = ( u - 8 )²
because: A² - 2 A B + B² = ( A - B )²
Answer: 64
"The diameter is 10, therefore radius = 5. If arc length = 10, then angle = 10/5 = 2 radians 2 radians = 2/2pi = 1/pi of a complete circle.
Area of circle = pi * r^2 = 25pi.
Area of sector = angle * area of circle = 1/pi * 25pi = 25"
