The radius of the big circle is equal to the diameter of the small circle. The sum of the areas of the two circles is 100 square centimeters. The area of the big circle is 100 square centimeters______ Square centimeter

The radius of the big circle is equal to the diameter of the small circle. The sum of the areas of the two circles is 100 square centimeters. The area of the big circle is 100 square centimeters______ Square centimeter

Let the radius of the big circle be r, then the radius of the small circle be R2, its area ratio is 4:1, and the area of the big circle is 100 × 45 = 80 (square centimeter)

How to find the area of a circle

Radius = R, diameter = L, l = 2R
Pai = pi = 3.14
Area s = Pai * R ^ 2 = Pai * L ^ 2 / 4

How to calculate the area of a circle? For example, how to calculate r = 0.2m? How to calculate d = 7cm?
Not formulas, but formulas

According to the formula, circle area = π * radius square
So only the radius is required
R = 0.2m, radius R is known, so area = π * 0.2 * 0.2 = 0.1256 m2
D = 7cm, so radius = half of diameter = 7 / 2 = 3.5cm
So area = π * 3.5 * 3.5 = 38.456 square centimeter

Calculation by factorization: 10000 of 252 ^ 2-248 ^ 2

=10000/(252+248)(252-248)
=10000/(500*4)
=10000/2000
=5

There are five different mathematics books and four different Chinese books. How many ways can we choose two mathematics books from them?
5 * 4 * 4 = 80. I think it should be (2 out of 5 combinations) * 4 = 40
Who can tell me what's wrong?

C (5,2) * C (4,2) = 40. According to the principle of multiplication, choose math books first, there are C (5,2) = 10 choices. Then choose Chinese books, there are C (4,1) = 4 choices. So there are 10 * 4 = 40 choices. This is not an arrangement, it should be right
There's repetition in 80
It should be 40
There's repetition in 80
Ask the teacher

One third of x plus 2 times 105 minus x difference equals 105 to solve the equation,
1/3x+2*(105-x)=105

The question asked by the landlord is very vague, is (1 / 3x + 2) * (105-x) = 105 or 1 / 3x + 2 * (105-x) = 105?

x(x^2-1)+2x^2 *(x+1）-3x*(2x-5)=x(3x^2-4x)-28

x(x^2-1)+2x^2 *(x+1）-3x*(2x-5)=x(3x^2-4x)-28;
x^3-x+2x^3+2x^2-6x^2+15x=3x^3-4x^2-28;
14x=-28.
x= -2.

One in nine plus two in nine plus three in nine plus four in nine equals one in nine times () = ()

+（10）=9/10

The equation 3: 8 = 24: x36x = 543x + 38x = 1214x-3 × 9 = 29

（1）3：8=24：x          3x=8×24       3x÷3=192÷3           x=64；（2）36x=543    54x=36×354x÷54=108÷54      x=2；（3）x+38x=121       118x=121  118x÷118=121÷118        x=88；（4）4x-3×9=29    4x-27+27=29+27       4x÷4=56÷4           x=14．

Reasonable calculation: 20 / 27 (5 / 6-1 / 18 + 7 / 24)

=20/27÷（15/18-1/18+7/24）
=20/27÷（14/18+7/24）
=20/27÷（56/72+21/72）
=20/27X72/77
=20/3X8/77
=160/231