That follows from the definition of the sine function. There are several equivalent definitions, but for example, it can be defined as the y-coordinate of the unit circle, as a function of the angle. You start measuring the circle from coordinates (1,0), and continue counterclockwise.
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This is because both sin definition is the length of line opposite to the angle divided by the angle side line. Since both lines are positive in the first quadrant, the sin value is positive.
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1st quadrant is +x and +y.
There's a mnemonic for this: All Students Take Calculus. Starting in the first quadrant, and moving counterclockwise until the last, give each quadrant the first letter of thos words in order. A represents all 3, s represents sine, t represents tangent, and c represents cosine. If the letter appears in a quadrant, it is positive there. If not, it is negative there.In quadrant 2, only sine is positive.
The coordinates must be as follows: First quadrant: positive, positive Second quadrant: negative, positive Third quadrant: negative, negative Fourth quadrant: positive, negative
That's Quadrant - I .
Quadrant one is the upper right quadrant, or where both X and Y are positive.