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Original poster

Help with a maths question please???

19 replies

ForkingBullshirt · 02/04/2019 20:52

Hi, so the question is in 2 parts.

DEF is an isosceles triangle. The longest side, DE, is 9cm and angle F is 95 degrees.
(a) Find the angles D and
(b) Use the sine rule to find the length of side EF.

I'm currently going through old exam papers in the run up to my first maths exam in many years!

I've completed this question but I'm unsure if I've got the correct answer. If anyone could help me out I'd be eternally grateful!

OP posts:
Original poster
ForkingBullshirt · 02/04/2019 20:55

Gah! part (a) should read, find the angles D and E.

OP posts:
StrongerThanIThought76 · 02/04/2019 21:09

D and E are 42.5°

Sculpin · 02/04/2019 21:10

Yes 42.5

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littlecabbage · 02/04/2019 21:12

I concur with 42.5.

camelfinger · 02/04/2019 21:14

6.09cm?

StrongerThanIThought76 · 02/04/2019 21:15

6.104cm

Original poster
ForkingBullshirt · 02/04/2019 21:16

yep.
I got the angles are 42.5 degrees but I got 6.9cm for the length of side DE...

OP posts:
Original poster
ForkingBullshirt · 02/04/2019 21:17

WooHOO!

I think I got it right!

Thanks everyone.

OP posts:
Sculpin · 02/04/2019 21:17

Length of EF = 4.5 / sine 47.5

Original poster
ForkingBullshirt · 02/04/2019 21:18

Nope. I'm an idiot...I got 6.1cm for the side

OP posts:
camelfinger · 02/04/2019 21:20

I think it is 6.1. Trying to do calcs on phone here.

Sculpin · 02/04/2019 21:22

I agree with 6.1

Gisel · 02/04/2019 21:24

The sum of the angles in a triangle is 180 degrees as far as I remember. Since it's isosceles, the two remaining angles, are both equal and must add up to 85 degrees. Therefore They would both be 42.5.

I can't remember Cos/Sin It was 24 years ago.

Can your dc discuss the cos/sin thing with the teacher if they can't understand yet? I was good at trig back in the day, not anymore.

StealthPolarBear · 02/04/2019 21:25

Someone tell me the formula? I can only remember the one for right angled triangles!

PatriciaHolm · 02/04/2019 21:26

In an isosceles triangle, 2 sides are of equal length. If DE is the longest, then DF and EF are the same and so are angles D and E, so in this case each would be 1/2(180-95) = 42.5

The Sine rule says that the 3 "length of a side/sine of the opposite angle" equations are equal. Here we have 9/sine 95 must equal length EF/sine 42.5.

9/sine 95 = 9.04, so length EF = 9.04Xsine42.5 = 6.1cm.

StealthPolarBear · 02/04/2019 21:26

Oh of course. Thanks

adviceatthislatestage · 02/04/2019 21:31

I remember learning this in trigonometry classes taken in 1981

Some old hags chins are hairy through old age =

Sine opposite/ hypotenuse
Cosine adjacent/hypotenuse
Tangent opposite/ adjacent

Still couldn't work out the answer of OPs 2nd question though Smile

StrongerThanIThought76 · 02/04/2019 22:15

Advice that's for right angled triangles. Still good maths though after nearly 40 years....Wink

Original poster
ForkingBullshirt · 03/04/2019 09:46

Oh Wow! Thanks so much.
This has really helped me.

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