mental math

Round 98 to the nearest 10. This is 100. Call this a. Calculate b which is a - our original number: b = 100 - 98 b = 2 We can use this formula: (a - b)^2 = a(a- 2b) + b^2 Given a = 100 and b = 2, we have: (100 - 2)^2 = 100(100 - 2(2)) + 2^2 98^2 = 100(96) + 4 98^2 = 9600 + 4 98^2 = 9,604

987 x 9 ------ Break apart 987: 900 + 80 + 7 So we have: 900 * 9 = 8100 80 * 9 = 720 7 * 9 = 63 Add them up and we get 8,883