The equation of motion is s = s = - 1 / 3T ^ 2 + 2T ^ 2-5 If the motion equation of the object is s = - 1 / 3T ^ 2 + 2T ^ 2-5, what is the instantaneous velocity of the object at t = 3?

The equation of motion is s = s = - 1 / 3T ^ 2 + 2T ^ 2-5 If the motion equation of the object is s = - 1 / 3T ^ 2 + 2T ^ 2-5, what is the instantaneous velocity of the object at t = 3?

s=5/3t^2-5=1/2at^2+vt-5
so,a=10/3,v=0,v'=at
so,t=3,v'=10/3*3=10m/s

The motion equation of a particle moving on the XY plane is x = 3T + 5, y = T2 + T-7,
No, I see

The parametric equation of particle trajectory: x = 3T + 5. (1) y = T & # 178; + T-7. (2)
From (1): T = (X-5) / 3 to (2): T = (X-5) / 3
Y = (X-5) & # / 9 + (X-5) / 3-7, that is, y is a quadratic function of X, so the trajectory is a parabola

A particle moves on the XY plane. The equation of motion is x = 3T + 5, y = T2 + T-7. It is proved that the trajectory of the particle is a parabola. Thank you for your help

The parameter equation of particle trajectory is: x = 3T + 5. (1) y = T2 + T-7. (2) from (1), t = (X-5) / 3 is substituted into (2), y = (X-5) 2 / 9 + (X-5) / 3-7, that is, y is a quadratic function of X, so the trajectory is a parabola

P is the point on the ellipse x ^ / 16 + y ^ / 9 = 1, F1 and F2 are the left and right focus of the ellipse respectively. If | Pf1 |. | PF2 | = 12, then the size of ∠ f1pf2 is?
A,30° B 60° C 120° D 150°

Let P (x, y), then
|Pf1 | = a + ex, | PF2 | = a-ex, where a = 4, B = 3, C ^ 2 = a ^ 2-B ^ 2 = 7, e = C / a = √ 7 / 4
From the known | Pf1 | * | PF2 | = 12
a^2-e^2x^2=12,
The solution is x ^ 2 = 64 / 7
So, | Pf1 | = 6, | PF2 | = 2 or | Pf1 | = 2, | PF2 | = 4,
Because | F1F2 | = 2C = 2 √ 7, from the cosine theorem,
cos∠F1PF2=(4+16-28)/(2*2*4)=-1/2
∠F1PF2=120°
Choose C

A gossip——
A person who offers an idea or opinion afterwards——

A gossip woman
A person who puts forward a method or opinion after the event

Find the trajectory equation of the midpoint of the chord passing through the origin on the circle x ^ 2 + y ^ 2-4x = 0

The midpoint m (x, y) of the chord OA passing through the origin
xA=2x,yA=2y
(xA)^2+(yA)^2-4xA=0
4x^2+4y^2-8x=0
x^2+y^2-2x=0

Practical problems of decimal in Volume 2 of grade 4

1. Xiaohong is 156 cm tall and Xiaofang is 1.52 m tall. How much is Xiaohong taller than Xiaofang? 2.50 kg of rapeseed can squeeze 15 kg of oil. According to this calculation, how many kg of 5 tons of rapeseed can squeeze oil? 3. Xiaoming's home is 1.5 km away from the school, Xiaonan's home is 1.60 km away from the school. Who's home is close to the school? How much is it? 4

A square with a circumference of 10 decimeters has an area of 1 square meter
Is that right or wrong? Why?

Side length: 10 △ 4 = 2.5m
Area: 2.5x2.5 = 6.25m2
This sentence is wrong

If a simple harmonic motion is made, the distance of the vibrating object must be two amplitudes within 1 / 2T

Yes, that's for sure

A 40 cm long, 30 cm wide rectangular cardboard, its four corners were cut off, side length of 5 cm square, and then make a no cover box
Finding volume

The height of the box is equal to the side length of the square, which is 5cm
The length of this box is 40-5 * 2 = 30 cm
The width of this box is 30-5 * 2 = 20 cm
The volume of this box is 30 * 20 * 5 = 3000 cubic centimeters