What is the remainder when the integer equal to QT2 is divided by 100, given that QU=933 and UT=40?
A number or a short expression. Spacing and $ signs are ignored.
Solution
Let O be the centre of the top face of the cylinder and let r be the radius of the cylinder. We need to determine the value of QT2. Since RS is directly above PQ, then RP is perpendicular to PQ. This means that △TPQ is right-angled at P. Since PQ is a diameter, then PQ=2r. By the Pythagorean Theorem, QT2=PT2+PQ2=n2+(2r)2=n2+4r2. So we need to determine the values of n and r. We will use the information about QU and UT to determine these values. Join U to O. Since U is halfway between R and S, then the arcs RU and US are each one-quarter of the circle that bounds the top face of the cylinder. This means that ∠UOR=∠UOS=90∘. We can use the Pythagorean Theorem in △UOR and △UOS, which are both right-angled at O, to obtain UR2=UO2+OR2=r2+r2=2r2 and US2=2r2. Since RP and QS are both perpendicular to the top face of the cylinder, we can use the Pythagorean Theorem in △TRU and in △QSU to obtain QU2=QS2+US2=m2+2r2 and UT2=TR2+UR2=(PR−PT)2+2r2=(QS−n)2+2r2=(m−n)2+2r2. Since QU=933, then QU2=92⋅33=2673. Since UT=40, then UT2=1600. Therefore, m2+2r2=2673 and (m−n)2+2r2=1600. Subtracting the second equation from the first, we obtain the equivalent equations m2−(m−n)2=1073 and m2−(m2−2mn+n2)=1073 and 2mn−n2=29⋅37 and n(2m−n)=29⋅37. Since m and n are integers, then 2m−n is an integer. Thus, n and 2m−n are a factor pair of 29⋅37=1073. Since 29 and 37 are prime numbers, the integer 1073 has only four positive divisors: 1, 29, 37, 1073. This gives the following possibilities: n=1,2m−n=1073,m=537; n=29,2m−n=37,m=33; n=37,2m−n=29,m=33; n=1073,2m−n=1,m=537. Since m>n, then n cannot be 37 or 1073. Since QU>QS, then m<933≈51.7. This means that n=29 and m=33. Since (m−n)2+2r2=1600, we obtain 2r2=1600−(m−n)2=1600−42=1584 and so QT2=n2+4r2=292+2(2r2)=841+3168=4009. The remainder when QT2 is divided by 100 is 9.
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