Let a = sinx, b = cosx, c = siny and d = cosy.
So ABCD = sinxcosxsinysy = [(1/2) sin2x] [(1/2) sin2y]
Therefore, taking appropriate x and y can make the minimum value of abcd-1/4.
See below. . .
The first teaching goal
1. Let students understand the meaning of solution ratio.
2. Make students maste