Cho hai đường tròn (O;R) và (O';r) cố định, tiếp xúc ngoài với nhau tại A (R>r). M là điểm thuộc đường tròn (O;R), N là điểm thuộc đường tròn (O';r) sao cho góc MAN= 90 độ a) Chứng minh OM // O'N
b) Chứng minh đường thẳng MN luôn đi qua một điểm cố định

Titus works at a hotel. part of his job is to keep the complimentary pitcher of water at least half full and always with ice. when he starts his shift, the water level shows 4 gallons, or 128 cups of water. as the shift progresses, he records the level of the water every 10 minutes. after 2 hours, he uses a regression calculator to compute an equation for the decrease in water. his equation is w –0.414t + 129.549, where t is the number of minutes and w is the level of water. according to the equation, after about how many minutes would the water level be less than or equal to 64 cups?