In △ ABC, the lengths of opposite sides of inner angles a, B and C are a, B and C respectively. If a2-c2 = 2B and sinacosc = 3cosasinc, then B = () A. 4B. 42C. 23D. 33

In △ ABC, the lengths of opposite sides of inner angles a, B and C are a, B and C respectively. If a2-c2 = 2B and sinacosc = 3cosasinc, then B = () A. 4B. 42C. 23D. 33

Sinacosc = 3cosasinc, using sine and cosine theorem, we get: aa2 + B2 − c22ab = 3cb2 + C2 − a22bc, and the solution is: 2 (a2-c2) = B2. ① because: a2-c2 = 2B, ② get: B = 4 from ①, we choose: a

The linear regression equation between Y and x ^ y = ^ BX + ^ a must pass the point: A. (0,0) B (x pull, 0) C (0, y pull) d (x pull, y pull)

The mean value must be on the regression line

Two problems of definite integral? Calculation
The interval is - 0.5 to 0.5 [(x * x) / (radical (1-x * x))] DX
The interval is 0 to 2 [1 / (radical (1 + 2 * x * x))] DX
The above two problems are solved by the method of substitution of definite integral

∫[x^2/√(1-x^2)]dx,[-0.5,0.5]
Let x = Sint and the integral range be [- π / 6, π / 6]
√(1-x^2)=cost,dx=costdt
∫[x^2/√(1-x^2)]dx,[-0.5,0.5]
=∫(sint)^2dt,[-π/6,π/6]
=∫[1-cos2t]/2dt,[-π/6,π/6]
=t/2-sin2t/4,[-π/6,π/6]
=π/12-√3/8-(-π/12+√3/8)
=π/6-√3/4
∫dx/√(1+2x^2),[0,2]
Let √ 2 * x = tank, then the integral range is [0, arctan (2 √ 2)]
√(1+2x^2)=sect,dx=(1/√2)(sect)^2dt
∫dx/√(1+2x^2),[0,2]
=(1/√2)∫sectdt,[0,arctan(2√2)]
=(1/√2)∫dt/cost,[0,arctan(2√2)]
=(1/√2)∫costdt/[1-(sint)^2],[0,arctan(2√2)]
=(1/√2)∫dsint/[1-(sint)^2],[0,arctan(2√2)]
=(1/2√2)∫[1/(1-sint)+1/(1+sint)]dsint,[0,arctan(2√2)]
=(1/2√2)∫[dln[(1+sint)/(1-sint)],[0,arctan(2√2)]
=(1/2√2)ln[(1+sint)/(1-sint)],[0,arctan(2√2)]
=(1/2√2)ln[(1+sinarctan2√2)/(1-sinarctan2√2)]
=(1/2√2)ln[(1+2√2/3)/(1-2√2/3)]
=(1/√2)ln(3+2√2)

A few math problems in sixth grade exercise book 2,
The number of white rabbits is one sixth less than that of black rabbits. What is the number of white rabbits? What is the number of black rabbits? What is the number of black rabbits? How many are the number of black rabbits more than that of white rabbits? What is the number of black rabbits in the total number of rabbits?
Must be correct, according to the sixth grade knowledge to answer, the answer is rewarded!
I have to have a formula. Pay attention to the formula!

(1)(6-1)/6=5/6
(2)6/(6-1)=6/5
(3)[6-(6-1)]/(6-1)=1/5
(4)6/[(6-1)+6]=6/11

It is known that the ellipse C takes F1 (- 2,0) F2 (2,0) as the focus and solves the elliptic equation (2) through P (- 5 / 2,3 / 2) (1) if the line L with slope 1 intersects the ellipse C
The equation for finding the straight line L at two points a and B and the circle with radius AB just passes through the midpoint of ellipse C
Take AB as diameter Sorry, wrong number

AB is the diameter of the circle. Thank you for your help

Please explain the area of polygon in Grade 5, thank you! (211:24:21)
There is a parallelogram vegetable field, de = EF = FC, GB = 1 / 3bd, the triangle GEF is cabbage, the area is 8 square meters, find the area of this parallelogram vegetable field? (this is really the fifth grade thinking innovation problem) how to draw the picture above

S (EFG) = 1 / 2S (DFG), so s (DFG) = 8 * 2 = 16 square meters,
S (DFG) = 1 / 2 DG * FP (P is the perpendicular foot of DB in F direction)
=1 / 2 (2 / 3dB) * (2 / 3cq) (q is the perpendicular of C to DB)
=4/9（1/2DB*CQ)
=4/9S(CDB)
=2 / 9s (parallelogram ABCD)
That is s (parallelogram ABCD) = 9 / 2S (DFG) = 9 / 2 * 16 = 72 square meters

Let f (x) be equal to ln (- the square of x-2x plus 3)
Find f (x) domain, range, monotone interval
I need a process! Thank you

From - X & # 178; - 2 x + 3 > 0
x²+2x-3＜0→﹙－3,1﹚
The domain of definition is (- 3,1)
The range of true number is (0,4) → range (- ∞, ln4]
Monotone intervals (- 3, - 1) are monotone increasing intervals
(- 1, + 1) is a monotone decreasing interval

How about the same
To do this:
Answer: 2.5 × 2=
0.5×10=
0.6×8=
2.1×2=
2.8×10=
0.7×0.8=

2.5x2 = 2x2.5, the rest is needless to say

As shown in the picture, Li Ming wants to cut a square board with an area of 25cm from another rectangular board. Can you help him find out the side length of the square board?
If Li Ming wants to cut two pieces of square cardboard with side length of 3cm diagonally to form a big square, can you help him find out the area of the big square? Is its side length an integer? If it is not an integer, then please estimate the value between the two integers,

As shown in the picture, Li Ming wants to cut a square board with an area of 25cm from another rectangular board. Can you help him find out the side length of the square board?
The side length of a square cardboard is 5cm
If Li Ming wants to cut two pieces of square cardboard with side length of 3cm diagonally to form a big square, can you help him find out the area of the big square? Is its side length an integer? If not, please estimate the value between the two integers
The area of this large square = √ 18 ×√ 18 = 18 square centimeters
Its side length is not an integer
It is estimated that the value of this side length is between 4cm and 5cm

How to show amplitude and period of sinusoidal alternating current phasor diagram
The phasor diagram doesn't move. The alternating current is changing all the time

Phasor is a complex number. It has amplitude and angle. It is similar to sine function
The amplitude is the amplitude of the sine and the amplitude is the phase of the sine
Why use complex number to express sine function? Because complex number is better for addition, subtraction, multiplication and division
According to Euler's formula e ^ IX = cosx + isinx, it can be concluded that the imaginary step of complex number is a sine function. When complex number is added or subtracted, the imaginary part of real part is added or subtracted separately, which does not affect each other. Therefore, complex number can be used to replace sine function to add or subtract each other (such as Kirchhoff's law of voltage and current)
As for the multiplication and division of imaginary numbers, such as U / I = JWL and U / I = - JXC, this is derived from the sine, and then compares the results of complex numbers, adding J. in addition to this place, phasor can be used to replace sine for multiplication and division. In other places, phasor can not be used, such as power calculation
In a word, phasor can be used instead of sine operation in addition and subtraction. It can also be used in impedance calculation. It can't be used in other times!