13. Certain radioactive materials decays at the rate constant of 2 percent Determine its half-life.?

15. Which of the following is an ellipse?

a. 5x^2+6y^2-7=0

b. 5x^2+6y^2+7=0

c. 5x^2-6y^2-7=0

16. Use the Shell method to find the volume of the solid generated by revolving about y=-2 the region on the first quadrant bounded by y=arcocsx , x=0 and y=0?

18. Use the Disk/Washer method to set up the integral to find the volume of the solid obtained by rotation about thex-axis, the region bounded by the curvesy= squre root x-2, y=0 and x=5?

20.Certain radioactive materials decays 25 percent in 3 years . Determine its annual decay rate constant?