Use the identity for sin(A+B) to find the exact value of sin 75.

From reading the question, we need to use the addition formula for sin(A+B)= sinAcosB+cosAsinB. So we need to think what are two common values for sine and cosine that we know and have exact answers for that also add up to 75? Our 'common angles' for sin and cosine are 30, 45 and 60, and we know that 30+45=75 so we can set A=30 and B=45. Plugging this into our formula we get:

sin(75)=sin(30+45)=sin(30)cos(45)+cos(30)sin(45)

Using our knowledge that sin(30)=1/2, cos(45)=sin(45)=sqrt(2)/2 and cos(30)=sqrt(3)/2, we can substitute them into the above formula:

sin(75)=1/2sqrt(2)/2+sqrt(3)/2sqrt(2)/2=(sqrt(2)+sqrt(2)*sqrt(3))/4

We can see that sqrt(2) is a common factor of the numerator so we can factorise giving us our answer:

sin(75)=sqrt(2)*(1+sqrt(3))/4

Answered by Ruth D. Maths tutor

3462 Views

See similar Maths A Level tutors

Related Maths A Level answers

All answers ▸

Find the cartesian equation of a curve?


find the coordinate of the maximum value of the function f(x) = 9 – (x – 2)^2


How do I simply differentiate and what does a differential mean?


At t seconds, the temp. of the water is θ°C. The rate of increase of the temp. of the water at any time t is modelled by the D.E. dθ/dt=λ(120-θ), θ<=100 where λ is a pos. const. Given θ=20 at t=0, solve this D.E. to show that θ=120-100e^(-λt)


We're here to help

contact us iconContact usWhatsapp logoMessage us on Whatsapptelephone icon+44 (0) 203 773 6020
Facebook logoInstagram logoLinkedIn logo

© MyTutorWeb Ltd 2013–2025

Terms & Conditions|Privacy Policy
Cookie Preferences