For any positive real number x,⌊x⌋ denotes the largest integer less than or equal to x. If ⌊x⌋⋅x=36 and ⌊y⌋⋅y=71 where x,y>0, what is x+y equal to?
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
For any positive real number x,⌊x⌋ equals the largest integer less than or equal to x and so ⌊x⌋≤x. In particular, ⌊x⌋⋅x≤x⋅x=x2. Thus, if ⌊x⌋⋅x=36, then 36≤x2. Since x>0, then x≥6. In fact, if x=6, then ⌊x⌋=⌊6⌋=6 and so ⌊x⌋⋅x=x2=36. Therefore, x=6. (Note that if x>6, then ⌊x⌋⋅x>6⋅6=36.) Also, since ⌊y⌋⋅y=71, then y2≥71. Since y>0, then y≥71≈8.43. Since y≥71≈8.43, then ⌊y⌋≥8. Suppose that ⌊y⌋=8. In this case, y=⌊y⌋71=871=8.875. Note that if y=871, then ⌊y⌋=8, so y=871 is a solution. Therefore, x+y=6+871=8119.
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