cosa 3 5 and a 2 , , so sina ? Why, the specific process. What do you think about similar topics?

Solution: Because.

cosa)^2+(sina)^2=1

So sina = (1-(cosa) 2) 4 5 because a (2 , so sina 0

The sine of the second quadrant angle is positive).

So sina 4 5

Remembering the symbols of the sine, cosine, and tangent of the four inner angles of the quadrant is the key to solving such problems: the first quadrant is all positive, the second quadrant is sine, the third quadrant is tangent, and the fourth quadrant is cosine. (In the two-three-four quadrants, except for one of the mentioned ones that are positive, the rest are negative, the cosecant is the same sign as the sine, the secant is the same sign as the cosine, and the cotangent is the same sign as the tangent).

Some trigonometric relations between angles must also be remembered:

cosa)^2+(sina)^2=1 ,tana=sina/cosa,..

Jiangsu Wu Yunchao wishes you progress in your studies.

1：cosa=-3\5

cosa)^2+(sina)^2=1

Gives sina = 4 5 or sina = -4 5

in accordance with a (2, second quadrant).

Then sina = 4 5

2: This kind of question is mainly to remember the positive and negative situations of sin, cos, tan, and cot in the one, two, three, and four quadrants.

Specifically, sin is positive in the first and second quadrants, and negative in the third and fourth quadrants.

COS is positive in quadrants 14 and negative in quadrants 2 and 3.

tan, cot is positive in the first and third quadrants and negative in the second and fourth quadrants.

Because a (2, so a is in the second quadrant, so sina>o, so sina = 4 5

To do this kind of problem, you only need to know the positive and negative of the triangle corresponding to the value of a in that quadrant, the first quadrant is positive, the second quadrant only sin is positive, and the third quadrant tan ctg is positive.

The fourth quadrant cos is positive.

Then it's good to find the corresponding value.

You can memorize several Pythagorean numbers for easy answers, 3, 4, 5, 3, 1, 2, 5, 12, 13

This is more commonly used, so you can only use sin 2a+cos 2a=1.

First of all, when a (2 , i.e. in the second quadrant, sina is positive.

There is sin 2a + cos 2a = 1

Thus sina = plus or minus root sign (1-cos 2a) = 4 5 or -4 5 should be sina is a positive number, so sina = 4 5

cosa=1-2(sina 2) 2=-1 5,sina Hanchang2) 2=3 5,5 4 a 2 3 2,a with fierce 2 in the third quadrant to talk about selling chops,sina 2<0, so sina 2=( 15) 5

sina + cosa) 2 = 1 + 2 sinacosa = 1 9 sinacosa = -4 9sina 3 cos 3 = (sina + cosa) 3-1-3 sinacosa (cosaa + sina) = 1 27-1-(-4 3) (1 3) = -26 27 + 12 27 = -14 27 (sina-cosa) 2 = 1-2 * (-4 9) = 17 9 sina-cosa = 17 3 or this reed- 17 3

cosa=1-2(sina farrow2) 2=-1 5,sina 2) 2=3 5,5 4 a 2 3 2,a 2 in the third quadrant, sina 2<0, so sina 2=( 15) is a pants auspicious 5