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These 2 questions confuse me.
Can anyone help with some 3D trigonometry?

These 2 questions confuse me. Can anyone help with some 3D trigonometry?-example-1
User Jeff Beagley
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1 Answer

14 votes
14 votes

Answer:

Q2) 29 degree as unrounded to nearest degree is 28.95 degree

Q3) 69 degree as unrounded to nearest degree is 68.56 degree

Explanation:

QU 2)

When they speak of plane we see ABCD and also see ABC

So we need the length of AB and BC to find the diagonal CA

AB^2 + BC^2 = CA^2

16.4^2 + 9.1^2 = sqrt 351.77

CA^2 = sqrt 351.77 = 18.8 cm

We know CG = 10.4cm

We identify the hypotenuse for ACG triangle

We do trig tan x = opp/adj for CGA angle

Tan x = tan-1 10.4/18.8 = 28.95099521 degree

Tan x = tan-1 18.8/10.4 = 61.04900479 degree

so we know one is much smaller than the other

We also know ACG angle is 90 degree and that angle from ABCD that meets line AG is the smaller angle.

Answer therefore must be 28.95 degree = or 29 degree

QU 3)

we are basically looking for angle where VB meets BC line or AVB meets ABC we have the slant length, so step 1 is find the height by first dividing square base by 2 then finding the height.

= 7.6/2 = 3.8 cm

Then Pythagoras

BV^2 - 1/2 BC = height

10.4^2 - 3.8^2 = height

Height = sq rt 93.72 =9.68090905 = 9.7cm

Which means V to midpoint VC = V to midpoint AB

They are the same and the midpoints are 90 degree angles.

To find the required angle for VB + BCmidpoint or we wont be able to determine the right angle hypotenuse.

We do the same as last question determine the hypotenuse and where the angle sought is is where we use the trig function = adj/hyp

Because if it was the midpoint angle then it would be opp/adj like the question 1 so this time its cos of x.

cos x = adj/hyp = cos-1 (3.8/ 10.4) = 68.5687455

Answer is 68.56 degree

The reason we show the height is so we can check by doing opp/hyp

= sin of x = sin-1 (9.68090905/3.8) = 23.11171135

and 90 -23.11171135 = 66.8882887

= 67 degree

So we go with the first one and assume 9.68 was already simplified to 9.7cm

= sin-1 (3.8/9.7) = 23 degree 90-23 = 67 degree

but when rounded to 10.4cm for slant we get the same

= sin-1 (3.8/10.4)

So we realise here trig functions -1 doesn't work on the same 90 degree angle for both lines that meet such 90 degree angle.

We try the sin-1 (10.4/ 9.68090905) = 68.5687455 = 69 degree

and that where the lines join away from the 90 degree angle we can always find true answer, and see it is a match with the first cos trig function we did.

This proves that cos line 1/line2 = sin line 1/line 2 are the same when the larger number is numerator for sin representing the hypotenuse slant for sin as shown and when the larger of the sides is numerator for cos di

and smallest side acts as denominator for both trig functions.

User Muricula
by
2.8k points
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