On Rectangular's birthday, Xiao Rectangular greeted the guests happily at the door.

"Bang bang!" The small rectangle quickly opened the door, and the visitor smiled and said, "I am a square, a relative of your family." "Why don't you know?" Asked the little rectangle. The square said, "If your father's area is long by wide, my area is the same, but my length and width are the same, so my area is side by side." Hearing this, the little rectangle said, "So that's it. Please come in! Please come in! "

Just entering the square, two strange guests came again. The little rectangle asked, "Are you a relative of my family, too?"

A visitor said, "My name is parallelogram, and my area should be multiplied by my height. Cut a piece from my left and fill it on the right, just like your rectangle! " "

Another tourist said, "My name is Triangle, and I am half of a parallelogram." He also made a gesture, "You see, when I spell it like this, it becomes a parallelogram, so my area is equal to the base times the height and then divided by 2."

"That's interesting. Please come in!" The small rectangle happily welcomed the parallelogram and triangle into the door.

"Bang bang!" Another guest knocked at the door. The little rectangle opened the door politely and said kindly, "Ah! Uncle Trapezoid, come to my house! " Trapezoid smiled and said, "Yes, when I spell two, it is also a parallelogram." . My area calculation formula is: (upper bottom+lower bottom) × height ÷2, right? "

"yes! Uncle trapezoid, please! " The small rectangle came into the room holding the trapezoid and was about to close the door. Suddenly, a song came: "Go around and wish Big Brother Rectangle a happy birthday, have a dessert and have a glass of champagne …" Looking back, the little rectangle turned out to be a chubby circle, and he was waddling towards his home. The little rectangle stopped and said, "Your circle has nothing to do with our rectangle. What are you doing here? "

"Ha ha ..." Round belly laughed. "Have you ever cut moon cakes, kid? As long as each of your knives passes through the center of the circle, if you put many small triangles together, it will become a rectangle like your father! Rectangular area equals length times width, length equals half circumference, width equals radius, so circular area equals half circumference times radius, which is simply: radius × radius× π ... "The circle was in perfect order, and the small rectangle was fascinated, so I forgot to invite Uncle Yuan into the house as soon as possible. At this time, the little rectangular father walked to the door and said with a smile, "Brother Yuan, come in quickly!" " All the guests are here! ""Well ... "Round came into the room with a small rectangle.

Square, parallelogram, triangle, trapezoid, circle * * * Happy birthday to rectangle.