I have explained it in detail in "The Dog Running in the Car". You'd better not look.

According to the principle of vector superposition:

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Illustrate the principle of vector superposition with examples:

The running speed of the train is v 1, and people on the train run on the train at the speed of v2 relative to the train.

Write v 1 as v (fire), v2 as v (human fire), and the speed of human relative to the earth as v (human land).

Then: V (human land) =v (human fire) +v (fire land).

Look at the characteristics of this formula: man ~ earth = man ~ fire+fire ~ earth.

= => Man ~ Earth = Man ~ (fire+fire) ~ Earth = Man ~ Earth

This is the basic formula for solving all relative motion problems in senior high school (in fact, it should be called vector superposition problem). In the university, we call it "the components are connected end to end and the total amount is sealed."

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It is not recommended to use the landlord's corner marking method. I recommend mine.

Moreover, speed is always a vector.