First, clever calculation. 1. 1+3+5+7+ + 17+ 19=( 1+ 19)*[( 19+ 1)/4]= 1002.72+66+75+63+69=72+67- 1 +72+3+67-4+67+2 = 72 * 2+67 * 3+3 =144+201= 3453. (1+3+5+7++2009)- If everyone gets four oranges, there will be 1 1 more oranges. If everyone is divided into six parts, it will be less 13. How many children are there? How many oranges are there? Method 1: Set X offspring, 4x+11= 6x-13, 24=2x, x= 12, 4x+1= 59 Method 2: (/. There are 59 oranges. 2. Buy 5 stools and 5 chairs * * and pay 100 yuan, buy 2 stools and 5 chairs * * and pay 6 1 yuan. What's the unit price of stools and chairs? Stool: (100-61) ÷ (5-2) =13; Chair: 100÷5- 13=7 stool unit price 13 yuan, stool unit price 7 yuan. There are 83 students in the third and fourth grades, including 5 girls, half boys taking part in the math competition, and the rest are equally divided. How many more boys are there than girls in this grade? Male: (83-5) ÷ 3× 2 = 52; Female students: 83-52=3 1, 52-3 1=2 1. Boys are 2 1 more than girls. 3. 15x28x33, 9x35x88, 12x7x 15. Decomposition factor: (1) 3× 5× 2× 2× 7× 3×11; ⑵3×3×5×7×2×2×2× 1 1; ⑶2×2×3×7× 1 1×3×5; (4) 2× 1 1× 5× 2× 3× 7. Obviously, the second formula is multiplied by 2. Therefore, the result of the second formula 9x35x88 is not equal to the results of other formulas. Fourth, fill in "greater than", "less than" or "equal to" in the first two questions and see what you can find. If you want to continue writing, can you still write? 99x 99+ 199(=) 100 x 100999 x999+ 1000 x 1000999999+ 19999 = 1000 x 100099999