Maths yr2 trig help

https://www.revisely.com/alevel/maths/edexcel/pure-questions/trigonometry/trigonometric-identities-2/6
I’m stuck on 6c), this is my work so far:

cos(15t - a) = -1
15t - a = 180°

So should I use my previous value of a, and solve for t? But the question explicitly asks for f(t) first…? Any help is welcomed. Thank you!!

Reply 1

mqb2766

Original post by mihyeb

For c) youre only interested in the min value of 3sin+4cos, so its almost a write down using part a) or pythagoras on 3,4. Then for d) youd want the time at which it occurs so 15t-a=180 as you say, using "a" from part a).