7. Find a formula for a function f(x, y, z) whose level surface f = 4 is a sphere of radius 2, centered at the origin. 8. Write the level surface x+y+ = 1 as the graph of a function f(x, y). 9. The balance B, in dollars, in a bank accounts depends on the amount deposited, A dollars, the annual interest rate r%, and the time t, in months since the deposit, so B = f(A,r,t). (a) Is f an increasing or decreasing function of A? Of r? Of t? (b) Interpret the statement f(1250, 1, 25) 1276. Give units. x2 10. Describe the level surface f(x, y, z) = = +2 z = 1 in words. 4 11. Describe in words the level surfaces of f(x, y, z) = sin(x + y + z). 12. Sketch and label level surfaces of g(x, y, z) = 1 x - y for g = 0,-1,-2.