SOLUTION: y varies directly as the square of x. If y=80 when x=20. Find y when x=90

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Question 760510: y varies directly as the square of x. If y=80 when x=20. Find y when x=90
Answer by ramkikk66(644) About Me  (Show Source):
You can put this solution on YOUR website!
y varies directly as the square of x. This means that the ratio of y to x^2 is a constant. Let us call it k. Mathematically,
%28y+%2F+x%5E2%29+=+k+
y = 80 when x = 20. Substituting for y and x in the above equation,
+80+%2F+20%5E2+=+k+ or
+k+=+80+%2F+400+=+0.2+
So when x = 90,
+y+%2F+90%5E2+=+k+=+0.2+
Therefore, +y+=+90%5E2+%2A+0.2+=+8100+%2A+0.2+ or +y+=+1620+