Question 624976
let me re-write the question which two tutors could not understand because of the student not using a proper notation, even it was more or less obvious what he meant:
Question 624950: 
If cos= 2 to the square root of 10/7 calculate the value of sin.

Should be posted this way: 
If cos = (2 x sq.root(10))/7 calculate the value of sin. (answer is a.)

a. 3/7
b. square root of 29/7
c. square root of 69/7
d. square root of 89/7
Found 2 solutions by Alan3354, jsmallt9: 
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The only thing obvious about it was that it made no sense.
Part of education is learning how to make sense, not speak gibberish.
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{{{cos(x) = 2sqrt(10)/7}}}
{{{sin(x) = sqrt(1 - cos^2(x))}}}
{{{sin(x) = sqrt( 1 - 40/49) = sqrt(9/49)}}}
sin = 3/7 (principal value)