According to the area of No.65438 +0, No.2 and No.3 plates, we can know that: S 1+2*S2=3*S3, namely: A? +2b？ =3ab .

(2) According to the answer in the question (1), it is known that 2 cards, 3 cards and 7 cards were used to prove the question.

The answer teacher suggested that if you can't find the train of thought at first, you can read all the questions first and find the answers from the words of the questions or the algebraic relationship provided by the answers.

For example, question: Is the area of card 1 a? What is the area of card No.2? The area of card 3 is ab. According to the algebraic expressions we have learned, calculate a by addition and subtraction? 、2b？ (1), we can know what kind of algebraic relationship exists between 3ab.

After understanding and solving the meaning in the question (1), the answer to question (2) is obvious.