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Geometry Difficulty 2.7 Junior Find the answer

What is the remainder when the integer equal to QT2 QT^2 is divided by 100, given that QU=933 QU = 9 \sqrt{33} and UT=40 UT = 40 ?

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let O O be the centre of the top face of the cylinder and let r r be the radius of the cylinder. We need to determine the value of QT2 QT^2 . Since RS RS is directly above PQ PQ , then RP RP is perpendicular to PQ PQ . This means that TPQ \triangle TPQ is right-angled at P P . Since PQ PQ is a diameter, then PQ=2r PQ = 2r . By the Pythagorean Theorem, QT2=PT2+PQ2=n2+(2r)2=n2+4r2 QT^2 = PT^2 + PQ^2 = n^2 + (2r)^2 = n^2 + 4r^2 . So we need to determine the values of n n and r r . We will use the information about QU QU and UT UT to determine these values. Join U U to O O . Since U U is halfway between R R and S S , then the arcs RU RU and US US are each one-quarter of the circle that bounds the top face of the cylinder. This means that UOR=UOS=90 \angle UOR = \angle UOS = 90^{\circ} . We can use the Pythagorean Theorem in UOR \triangle UOR and UOS \triangle UOS , which are both right-angled at O O , to obtain UR2=UO2+OR2=r2+r2=2r2 UR^2 = UO^2 + OR^2 = r^2 + r^2 = 2r^2 and US2=2r2 US^2 = 2r^2 . Since RP RP and QS QS are both perpendicular to the top face of the cylinder, we can use the Pythagorean Theorem in TRU \triangle TRU and in QSU \triangle QSU to obtain QU2=QS2+US2=m2+2r2 QU^2 = QS^2 + US^2 = m^2 + 2r^2 and UT2=TR2+UR2=(PRPT)2+2r2=(QSn)2+2r2=(mn)2+2r2 UT^2 = TR^2 + UR^2 = (PR - PT)^2 + 2r^2 = (QS - n)^2 + 2r^2 = (m - n)^2 + 2r^2 . Since QU=933 QU = 9 \sqrt{33} , then QU2=9233=2673 QU^2 = 9^2 \cdot 33 = 2673 . Since UT=40 UT = 40 , then UT2=1600 UT^2 = 1600 . Therefore, m2+2r2=2673 m^2 + 2r^2 = 2673 and (mn)2+2r2=1600 (m - n)^2 + 2r^2 = 1600 . Subtracting the second equation from the first, we obtain the equivalent equations m2(mn)2=1073 m^2 - (m - n)^2 = 1073 and m2(m22mn+n2)=1073 m^2 - (m^2 - 2mn + n^2) = 1073 and 2mnn2=2937 2mn - n^2 = 29 \cdot 37 and n(2mn)=2937 n(2m - n) = 29 \cdot 37 . Since m m and n n are integers, then 2mn 2m - n is an integer. Thus, n n and 2mn 2m - n are a factor pair of 2937=1073 29 \cdot 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=1,2mn=1073,m=537 n = 1, 2m - n = 1073, m = 537 ; n=29,2mn=37,m=33 n = 29, 2m - n = 37, m = 33 ; n=37,2mn=29,m=33 n = 37, 2m - n = 29, m = 33 ; n=1073,2mn=1,m=537 n = 1073, 2m - n = 1, m = 537 . Since m>n m > n , then n n cannot be 37 or 1073. Since QU>QS QU > QS , then m<93351.7 m < 9 \sqrt{33} \approx 51.7 . This means that n=29 n = 29 and m=33 m = 33 . Since (mn)2+2r2=1600 (m - n)^2 + 2r^2 = 1600 , we obtain 2r2=1600(mn)2=160042=1584 2r^2 = 1600 - (m - n)^2 = 1600 - 4^2 = 1584 and so QT2=n2+4r2=292+2(2r2)=841+3168=4009 QT^2 = n^2 + 4r^2 = 29^2 + 2(2r^2) = 841 + 3168 = 4009 . The remainder when QT2 QT^2 is divided by 100 is 9.

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