# Precalc

• co' bai nay giai hoai ko duoc, can u guys help ???????

CosB/(1 - tanB) + sinB/(1 - cotB) = sinB+cosB

Another one:

Sin(x + (3π/2) + sin((3π/2)- x) = -2cosx

ko biet ai con nho ko nua, ....thanks though...

• CosB/(1 - tanB) + sinB/(1 - cotB) = sinB+cosB .. prove left side equal right side use tanB = sinB/cosB & cotB = cosB/sinB
left side
((cosB)^2 - (sinB)^2 )/(cosB - sinB) = (cosB-sinB)(cosB+sinB)/(cosB-sinB) = cosB + sinB

second problem use formula
sinA + sinB = 2sin((A+B)/2)*cos((A-B)/2) ... have fun • hey mltr !
Van ko hieu cho nay
((cosB)^2 - (sinB)^2 )/(cosB - sinB)
tai sao o problem nó đang plus, why you changed it to minus ?????
i thought it still cosB^2 + sinB^2 / ..... chu ???

• because 1-cotB = (sinB-cosB)/sinB ... need to multiply by -1 to get cosB-sinB ==> you get -sin^2 • wow, khâm phục wá, học tới toán mấy rùi mà giỏi thía.....
Cám ơn nhiều nha, nhưng cái problem thứ 2, mình ko hiểu tại sao dùng fomula because
sin((x Plus 3n/2)) + sin ((3n/2 minus)) mà, theyr not the same......i don't think we can use sinA plus sinB ????
but thanks alot for helping me out.....

• emyeuanht2001 wrote:
Cám ơn nhiều nha, nhưng cái problem thứ 2, mình ko hiểu tại sao dùng fomula because
sin((x Plus 3n/2)) + sin ((3n/2 minus)) mà, theyr not the same......i don't think we can use sinA plus sinB ????
but thanks alot for helping me out.....

why not? Treat the whole thing inside () as A and/or B ... the formula is just general. or you can do the ... long way by expand sin(a+b) and sin(a-b) ... hehe

• long way nàh • hehé .. why not, just the way to practice problem solving  • I am so dud with this crap .. I cant even remember tan= gì nũa • TaK wrote:I am so dud with this crap .. I cant even remember tan= gì nũa haha .. I would like to forget them too, but after PE exams .. kaka .. too lazy to take it ... don't want to reviews (

• nhớ dai ghê , mltr zữ wa , thui zọt lolz • thanks mltr..............

• moving on to trig, next is sin cos tan just like urs ~_~

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