Monday

November 24, 2014

November 24, 2014

Posted by **Sarah** on Wednesday, May 26, 2010 at 9:39pm.

- math -
**Reiny**, Wednesday, May 26, 2010 at 9:55pmOK, B+C = 180-A

so

LS = 2sin(B+C)sinA

= 2sin(180 - A)sinA

= 2(-sinA)sinA

= -2 sin^2 A

RS = 1 - cos 2A

= 1 -(2sin^2 A - 1)

= -2sin^2 A

= LS

- math -
**Mike**, Wednesday, May 26, 2010 at 10:00pmin part 1, how does sin(180-A) equal -sinA from line 4-5

- Mike you are right - math -
**Reiny**, Wednesday, May 26, 2010 at 10:12pmActually I have 2 errors in my solution

1.

sin(180-A) = sinA , not -sinA

e.g. sin (150) = sin(180-150) = sin 30

2. cos 2A = 1 - 2sin^2 A, I had it backwards

so here is the correct solution

LS = 2sin(B+C)sinA

= 2sin(180 - A)sinA

= 2(sinA)sinA

= 2 sin^2 A

RS = 1 - cos 2A

= 1 -(1 - 2sin^2 A)

= 2sin^2 A

= LS

sorry about that

**Answer this Question**

**Related Questions**

math - how do you solve this.prove: cos2A plus cosA divided by sin2A minus sinA ...

trig - Prove: 1/cos2A+sin2A/cos2A=sinA+cosA/cosA-sinA

trig - Prove: 1/cos2A+sin2A/cos2A=sinA+cosA/cosA-sinA

Trig - Please help with this one. I've worked it out in so many ways and can't ...

MathS triG - 1 .If tanA=1/3 and tanB=1/7 (both A and B are acute),calculate ...

maths Pls help trig - 1 .If tanA=1/3 and tanB=1/7 (both A and B are acute),...

further Mathematics - Let A=(cosA -sinA) (sinA cosA) show that A^2=(cos2A sin2A...

Math - Trig - Double Angles - Prove: sin2x / 1 - cos2x = cotx My Attempt: LS: = ...

Math - In any angle ABC prove tht the perimeter = a/Sin A(Sin A + Sin B + Sin C...

Precal/trig - sina=3/5, pi/2<a<pi evaluate cos2a