WAEC Past Questions and Answers - Page 6

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26.

A committee consists of 6 boys and 4 girls. In how many ways can a sub-committee consisting of 3 boys and 2 girls be formed if one particular boy and one particular girl must be on the sub-committee?

A.

120

B.

80

C.

56

D.

30

Correct answer is D

The required committee can be formed in 

(a boy and a girl are already part of the committee so we need two more boys and a girl).

\(^{5}C_{2}\) * \(^{3}C_{1}\) = \(\frac{5 * 4*3*2*1}{3* 2 * 2*1}\) * \(\frac{ 3* 2* 1}{ 2 * 1}\)

10  * 3 = 30ways.

27.

The first term of an AP is 4 and the sum of the first three terms is 18. Find the product of the first three terms

A.

292

B.

272

C.

192

D.

172

Correct answer is C

Using the sum of an AP, S\(_n\) = \(\fra{n}{2}\) [ 2a + (n - 1)d]

S\(_3\) = \(\fra{3}{2}\) [ 2a + (3 - 1)d]

18 = \(\fra{3}{2}\) [ 2a + 2d]

2a + 2d = 12

a = 4

2(4) + 2d = 18 --> 8 + 2d = 12

2d = 4; d = 2

a = 4: a + d

=  4 + 2 = 6

a + 2d = 4 + 2]2] 

= 8

product of the terms = 4 * 6 * 8 = 192

28.

If sin x = \(\frac{12}{13}\) and sin y = \(\frac{4}{5}\), where x and y are acute angles, find  cos (x + y)

A.

\(\frac{48}{65}\)

B.

\(\frac{13}{15}\)

C.

\(\frac{-33}{65}\)

D.

\(\frac{-48}{65}\)

Correct answer is C

From Pythagoras' theorem

when sin x = \(\frac{12}{13}\), cos x = \(\frac{5}{13}\)

Using cos (x + y) = cosx cosy - sinxsiny

\(\frac{5}{13}\) * \(\frac{3}{5}\) - \(\frac{12}{13}\) * \(\frac{4}{5}\)

= \(\frac{-33}{65}\)

29.

If ( 1- 2x)\(^4\) = 1 + px + qx\(^2\) - 32x\(^3\) + 16\(^4\), find the value of (q - p)

A.

-32

B.

-16

C.

16

D.

32

Correct answer is D

( 1- 2x)\(^4\) = 1 + px + qx\(^2\) - 32x\(^3\) + 16\(^4\) 

compared with:  1 - 8x + 24x\(^2\) - 32x\(^3\) + 16\(^4\)

q = 24 and p = -8

(q - p) = 24 - [-8] = 32

30.

Given that F\(^1\)(x) = x\(^3\)√x, find f(x)

A.

\( \frac{2x^{9/2}}{9} + c \)

B.

\( 2x^{9/2} + c \)

C.

\( \frac{2x^{5/2}}{5} + c \)

D.

\( x^4 + c \)

Correct answer is A

F1 (x) = x\(^3\) √x = x\(^{7/2}\)

F(x) = \(\frac{x^{7/2 +1}}{7/2 + 1}\) + c

= \(\frac{x^{9/2}}{9/2}\)

= \(\frac{2x^{9/2}}{9}\) + c