Explain cos2a = 2cos & # 178; A-1

Explain cos2a = 2cos & # 178; A-1

The square of 4sin is B + c-cos2a = 7 / 2, so why is the square of 2 [1-cos (B + C)] - 2cos a + 1 = 7 / 2
I want steps and ideas

The formula can be reversed & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; Cos ^ 2 (a) is the square of cosa
According to the known conditions, combined with the formula cos2a = cos ^ 2 (a) - Sin ^ 2 (a) = 2cos ^ 2 (a) - 1 = 1-2sin ^ 2 (a), it can be concluded that:
The square of 4sin B + c-cos2a = 4sin ^ 2 (a) - 1 + 2Sin ^ 2 (a) = 7 / 2
Then 6sin ^ 2 (a) = 9 / 2 & nbsp; & nbsp; sin ^ 2 (a) = 3 / 4 & nbsp; & nbsp; & nbsp; & nbsp; then & nbsp; Cos ^ 2 (a) = 1 / 4 & nbsp; that is, cosa = 1 / 2
Let's look at the square a + 1 of & nbsp; 2 [1-cos (B + C)] - 2cos;
2[1-cos(B+C)]=2(1+cosA)
Then we can get the square of & nbsp; & nbsp; 2 [1-cos (B + C)] - 2cos a + 1 = 2 (1 + COSA) - cos ^ 2 (a) + 1 & nbsp; & nbsp;
The solution can be obtained by substituting cosa = 1 / 2 & nbsp
The above is the general idea

1-cos2a / cos2a reduction. It is better to have a process. Cos2a is the square of COS

=Sin2a / cos2a = tan2a ask: why is it equal to tan ^ 2? Formula? Find the formula... Answer: sin / cos = tan

There are 40 students in a class, two fifths of them are women. Later, several girls were transferred. Now, women account for seven fifths of the class. How many girls are transferred
It's best to use the equation

Let's have X girls
40×2/5+x=7/15×(40+x)
x=5
A: five more girls

Junior high school square root addition and subtraction method
（ 2√2+√32 ）÷√2

(2√2+√32)÷√2
＝(2√2+4√2)÷√2
＝2+4
＝6
Teach you a good ah ~ general square root addition and subtraction, first with the number of roots to the simplest, and then observe, it's easy to say

A, B and C go shopping together. One half of a's spending is equal to one third of B's spending, and three fourth of B's spending is equal to three fifth of C's spending
They spent 98 yuan more. How much did they spend

Hello
A: B = 1 / 3:1 / 2 = 2:3 = 8:12
B: C = 3 / 5:3 / 4 = 4:5 = 12:15
A: B: C = 8:12:15
They spend money together
98÷（15-8）×（8+12+15）
=98÷7×35
=490 yuan
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It is known that the square root of 2a-1 is positive or negative 3, and the arithmetic square root of B + 2 is 4

The square root of 2a-1 is plus or minus 3
2a-1=9
a=5
The arithmetic square root of B + 2 is 4
b+2=16
b=14
a+b=19

The ratio of number a to number B is 4:5, and the ratio of number B to number C is 6:7. What is the ratio of three numbers a, B and C?

A: B = 4:5 = 24:30
B: C = 6:7 = 30:35
A: B: C = 24:30:35

(2012 · Dazhou) if the solution of the binary linear equations 2x + y = 3K − 1x + 2Y = − 2 satisfies x + Y > 1, then the value range of K is______ ．

2X + y = 3K − 1, ① x + 2Y = − 2, ②, ① - ② × 2, y = - k-1; substituting y = - k-1 into ②, x = 2K, ∵ x + Y > 1, ∵ 2k-k-1 > 1, the solution is k > 2

80% of a number is equal to 5 / 6 of 120. What is the number? Comprehensive formula

120×5/6÷80%=125