Arectangular box has length 9 inches, width 7 inches, and a height of 15 inches. find the angle between the diagonal of the box and the diagonal of its base. the angle should be measured in radians.

0.92085175 radians  
Let's first calculate the length of the diagonal of the base. Using the Pythagorean theorem:
 b = sqrt(9^2 + 7^2) = sqrt(81 + 49) = sqrt(130) = 11.40175425 
 Now the length of the diagonal to the box. Once again, using the Pythagorean theorem:
 d = sqrt(15^2 + 11.40175425^2) = sqrt(225 + 130) = sqrt(335) = 18.84144368 
 We now have a right triangle where we know the lengths of all three sides. The lengths are:
 a = 15
 b = sqrt(130) ≠11.40175425
 c = sqrt(335) ≠18.84144368
 
 we want the angle opposite to side a. So
 tan(A) = 15/11.40175425 = 1.315587029
 A = atan(1.315587029)
 A = 0.92085175 radians