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Problems in Mathematical Analysis.pdf - pwp.net.ipl.pt

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Sec. 6]_Derivative <strong>in</strong> a Given Direction_19S<br />

1869. Show that the function<br />

w = f(u, v),<br />

where u = x + at, v = y + bt satisfy the equation<br />

dw dw ,<br />

dT= a dt<br />

1870. Show that the function<br />

satisfies the equation + = J<br />

1871. Show that the function<br />

satisfies the equation x-^ + y -^ = j<br />

1872. Show that the function<br />

2<br />

satisfies the equation (A-<br />

, dw<br />

y 2<br />

) ^- + xy-^ =<br />

1873. The side of a rectangle x -^20 m <strong>in</strong>creases at the rate<br />

of 5 m/sec, the other side f/ = 30 m decreases at 4 m/sec. What<br />

is the rate of change of the perimeter and the area of the rect-<br />

angle?<br />

1874. The equations of motion of a material po<strong>in</strong>t are<br />

What is the rate of recession of this po<strong>in</strong>t from the coord<strong>in</strong>ate<br />

orig<strong>in</strong>?<br />

1875. Two boats start out from A at one time; one moves<br />

northwards, the other <strong>in</strong> a northeasterly direction. Their velocities<br />

are respectively 20 km/hr and 40 km/hr. At what rate does<br />

the distance between them <strong>in</strong>crease?<br />

Sec. 6. Derivative <strong>in</strong> a Given Direction and the Gradient of a Function<br />

1. The derivative of a function <strong>in</strong> a given direction. The derivative of a<br />

function z = /(#, y) <strong>in</strong> a given direction / = PP, is<br />

7- 1900

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