Why is cos2a = - 7 / 25 If cosa = 3 / 5? Why is this formula 2cos squared A-1

Why is cos2a = - 7 / 25 If cosa = 3 / 5? Why is this formula 2cos squared A-1

2cos^2a-1=cos2a=2*(3/5)^2-1=-7/25

What can 1-cos2a become? The square of 1-2cos + 1 can simplify why? Urgent!

1-cos2a=1-(1-sin²a)=sin²a

When a batch of goods is to be transported to a certain place, the owner of the goods is going to rent two kinds of trucks from the motor transport company. The situation of renting this kind of trucks in the past two times is shown in the table. Now he rents five kinds of trucks from the company and six kinds of trucks from the company. How many tons of this batch of goods are there?
The first time, the second time
Class a goods vehicles (vehicles) 3 2
Type B freight cars (4 / 3)
Cumulative tonnage (ton) 36 26

Set X tons for type a vehicle and Y tons for type B vehicle,
Then 3x + 4Y = 36
2x+3y=26
x=4,y=6
This shipment is 4 * 5 + 6 * 6 = 56 tons

Only x = 0.5 is the solution of the equation 3M + 8x = 0.5 + X about X. find the equation m + 2x = 2m-3x about X

X = 0.5 is the solution of the equation 3M + 8x = 0.5 + X about X
So:
3m+8*0.5=0.5+0.5
=>
m=-1
-1+2x=2*(-1)-3x
The solution is as follows
x=-1/5

A and B have two barrels of oil. A weighs 90 kg. If 1 / 5 of a barrel is poured out to B barrel, the weight of the two barrels of oil is equal. B barrel of original oil is () kg

90 * 1 / 5 = 18 (kg)
90-18 * 2 = 54 (kg)
A and B have two barrels of oil. A weighs 90 kg. If 1 / 5 of a barrel is poured out to B barrel, the two barrels of oil have the same weight. B barrel of original oil is (54) kg

Quadratic function y = x ^ 2 + MX + N, if M + n = 0, then its image must pass through the point

Let x = 1, y = 1 & # 178; + m + n = 1 + 0 = 1
So the image must pass through the point (1,1)
Answer: (1,1)

There are 20 boys and 30 girls, and boys account for about 30% of girls______ %Boys make up 50% of the class______ %There are more girls than boys______ %．

20 ／ 30 ≈ 0.667 = 66.7%; 20 ／ (20 + 30) = 20 ／ 50 = 0.4 = 40%; (30-20) ／ 20 = 10 ／ 20 = 0.5 = 50%; answer: male students account for 66.7% of female students, male students account for 40% of the class, and female students account for 50% more than male students

If A-B = 2, B-C = 3, then A2 + B2 + C2 AB BC ca=______ ．

∵ A-B = 2 ①, B-C = 3 ②, ∵ ① + ②: a-c = 5, ∵ A2 + B2 + c2-ab-bc-ca = 12 (2A2 + 2B2 + 2c2-2ab-2bc-2ca) = 12 [(a2-2ab + B2) + (a2-2ac + C2) + (b2-2bc + C2)] = 12 [(a-b) 2 + (A-C) 2 + (B-C) 2] = 12 × [22 + 52 + 32] = 12 × 38 = 19

There are 42 students in class 3 of grade 5, of which the number of boys is 1.1 times that of girls. How many boys and girls are there

There were 18 women and 24 men
X women
There are 24: (18 + x) = 6:5
We get x = 2
2 girls
Now there are 42 + 2 = 44 people

On the quadratic equation KX square of X - (4K + 1) 10K + 3 = 0, K is a non-zero integer. (1) prove that the equation has two unequal real roots

（4k+1）^2-4k(3k+3)
=16k^2+8k+1-12k^2-12k
=4k^2-4k+1
=(2k-1)^2
Because K is a nonzero integer
So (2k-1) ^ 2 > 0
So the equation has two unequal real roots