(1) , 1-xy + 4 / 4 x square y (2), 38.9-2 × 38.9 × 48.9 + 48.9 square (3), 34 square + 34 × 32 + 16 square

(1) , 1-xy + 4 / 4 x square y (2), 38.9-2 × 38.9 × 48.9 + 48.9 square (3), 34 square + 34 × 32 + 16 square

We should be good at observing the relationship between the terms of the formula, and be familiar with various forms of squares and the split form of squares. It's easy to be familiar with both forward and backward deduction. 1) 1-xy + x ^ 2Y ^ 2 / 4 = 1-xy + (XY / 2) ^ 2, which is a complete square form. The original formula = 1-2 * (XY / 2) + (XY /