# Maths

Given that sinA=2/5 and that A is obtuse,find the value of cosA.

π 0 π 0 π 25
yesterday at 4:18am
sinA = 2/5, so
cosA = β21/5
But A is in QII, so cosA = -β21/5

π 0 π 0
posted by oobleck
yesterday at 6:51am
Why not sqrt(5^2-2^2)/5 ?

1. 👍
2. 👎
3. 👁
1. oobleck gave you the correct answer of cosA = -β21/5

You asked, Why not sqrt(5^2-2^2)/5 ?
well, isn't sqrt(5^2-2^2)/5 = β21/5 ?

The reason cosA is negative???
you said angle A is obtuse, so it must be between 90 and 180Β°, that is,
it must be an angle in the 2nd quadrant.
You should know the CASE rule, and according to that the cosine of a

1. 👍
2. 👎
👤
Reiny

## Similar Questions

1. ### Math

if sinA+sinB=x and cosA-cosB=y then find tan[(A-B)/2]

2. ### Mathematics

The point P(-4,9) is on the terminal arm of β A (a) Find the primary trigonometric ratios (sinA, cosA, tanA) in exact values. (b) Determine the measure of β A to the nearest degree.

3. ### math

sinA+cosA=p ,secA+cosecA=q (cosA)^2/[2-2(sinA)^2-(cosecA)^2]=p^2/(2(p^2)-q^2

4. ### maths

if sinA+cosA=1/5, then a value of tanA/2 satisfies which of the equation? a. 2x^2-x-1 b.x^2-2x-3=0 c.2x^2-5x-3=0 d. 2x^3-7x^2+2x+3=0

1. ### Trignometry

An isosceles triangle has two sides of length w that make a 2Γ‘-degree angle. Write down two different formulas for the area of this triangle, in terms of w and Γ‘ (Greek βalphaβ). By equating the formulas, discover a relation

2. ### Math

prove that sinA/(1+cosA) + (1+cosA)/(sinA) = 2cosecA

3. ### trigonometry

Prove : 1 + Sin2A/ Cos2A = CosA + SinA / CosA- SinA

4. ### maths

If a cosA - b sinA = c . Prove that a sinA + b cosA = _+βa^2 + b^2 - c^2

1. ### Trigonometry

If sinA + sinB = a and cosA + cosB = b, find the value of tanA-B/2

2. ### maths

Prove SinA/1-CosA = CosecA + CotA

3. ### math

12. IF 1/SinA+1/CosA=1/SinB+1/CosB ,PROVE THAT: Cot{(A+B)/2}=TanA.TanB

4. ### Math

Find angle A if it is greater than or equal to 0 degrees and smaller than or equal to 180 degrees. round to the nearest tenth of a degree, if necessary. a) sinA = 0.5 b) cosA = -0.5 e) cosA = 0.4561 g) sinA = 0.5736 PLEASE HELP