jackco

7. Is XY<1? (1) XYZ=1, (2) XYZ^2<1 The clue is that the 1)and 2) are combined, you can get "z>1", and add 1)xy=1/z, you can reach"01, xy<1, you can get XY

jackco

16. 714前6的倍数和
[确认]:3*119*120
you can see the old jj. The stem lacks a formula S=n(n+1)/2, according to the formula, you can get 6*119*(119+1)/2, equal to 3*119*120.

jackco

Attention: xyz那题, the correct condition 2) is xyz^2>1, not <1.

jackco

27 一个group中包括玛丽, 有两个人可得到free ticket. 不包括玛丽,得到free ticket 的组合有36 种, 问group 共有多少人 [ 10]
[确认]10
因为C（x,2)=36,得x=9，再加上马丽就是10个人罗。

4 Mars中心到一卫星的距离5800 miles,Mars表面到卫星的距离是3700miles，问Mars的周长。
[确认]4200

27), the correct expression is C(x-1,2)=36, then you get (x-1)(x-2)/2=36,x=10 or -7,choose x=10.

4) the answer should be 13200, because 4200pai=4200*22/7=13200.