第9章代数表达式和恒等式-练习9.5 |套装1
问题5:表明:
(i)(3x + 7) 2 – 84x =(3x – 7) 2
解决方案:
L.H.S. = (3x + 7)2 – 84x
= 9x2 + 42x + 49 – 84x
= 9x2 – 42x + 49
= (3x – 7)2
= R.H.S.
L.H.S. = R.H.S.
(ii)(9p – 5q) 2 + 180pq =(9p + 5q) 2
解决方案:
LHS = (9p – 5q)2 + 180pq
= 81p2 – 90pq + 25q2 + 180pq
= 81p2 + 90pq + 25q2
RHS = (9p + 5q)2
= 81p2 + 90pq + 25q2
LHS = RHS
(iii)(4/3 m – 3/4 n) 2 + 2mn = 16/9 m 2 + 9/16 n 2
解决方案:
LHS = (4/3 m – 3/4 n)2 + 2mn
= 16/9m2 + 9/16n2 – 2nm + 2mn
=16/9 m2 + 9/16 n2
= RHS
LHS = RHS
(iv)(4pq + 3q) 2 –(4pq – 3q) 2 = 48pq 2
解决方案:
LHS = (4pq + 3q)2 – (4pq – 3q)2
= 16p2q2 + 24pq2 + 9q2 – 16p2q2 + 24pq2 – 9q2
= 48pq2
RHS = 48pq2
LHS = RHS
(v)(a – b)(a + b)+(b – c)(b + c)+(c – a)(c + a)= 0
解决方案:
LHS = (a – b) (a + b) + (b – c) (b + c) + (c – a) (c + a)
= a2 – b2 + b2 – c2 + c2 – a2
= 0
= RHS
问题6:使用身份进行评估。
(i)71²
解决方案:
712 = (70+1)2
Using formula (a + b) 2 = a2 + b2 + 2ab
= 702 + 12 + 140
= 4900 + 140 +1
= 5041
(ii)99平方米
解决方案:
99² = (100 -1)2
Using formula (a – b) 2 = a2 + b2 – 2ab
= 1002 + 12 – 200
= 10000 – 200 + 1
= 9801
(iii)102 2
解决方案:
1022 = (100 + 2)2
Using formula (a + b) 2 = a2 + b2 + 2ab
= 1002 + 400 + 22
= 10000 + 400 + 4
= 10404
(iv)998 2
解决方案:
9982 = (1000 – 2)2
Using formula (a – b) 2 = a2 + b2 – 2ab
= 10002 – 4000 + 22
= 1000000 – 4000 + 4
= 996004
(v)5.2²
解决方案:
5.22 = (5 + 0.2)2
Using formula (a + b) 2 = a2 + b2 + 2ab
= 52 + 2 + 0.22
= 25 + 2 + 0.4
= 27.4
(六)297×303
解决方案:
297 × 303
= (300 – 3 ) (300 + 3)
Using formula (a + b) (a – b) = a2 – b2
= 3002 – 32
= 90000 – 9
= 89991
(vii)78×82
解决方案:
78 × 82
= (80 – 2) (80 + 2)
Using formula (a + b) (a – b) = a2 – b2
= 802 – 22
= 6400 – 4
= 6396
(viii)8.9 2
解决方案:
8.92= (9 – 0.1)2
Using formula (a – b) 2 = a2 + b2 – 2ab
= 92 – 1.8 + 0.12
= 81 – 1.8 + 0.01
= 79.21
(ix)10.5×9.5
解决方案:
10.5 × 9.5 = (10 + 0.5) (10 – 0.5)
Using formula (a + b) (a – b) = a2 – b2
= 102 – 0.52
= 100 – 0.25
= 99.75
问题7:使用a 2 – b 2 =(a + b)(a – b),找到
(i)51 2 – 49 2
解决方案:
512 – 492
= (51 + 49) (51 – 49)
= 100 × 2
= 200
(ii)(1.02) 2 –(0.98) 2
解决方案:
(1.02)2 – (0.98)2
= (1.02 + 0.98) (1.02 – 0.98)
= 2 × 0.04
= 0.08
(iii)153 2 – 147 2
解决方案:
1532 – 1472
= (153 + 147) (153 – 147)
= 300 × 6
= 1800
(iv)12.1 2 – 7.9 2
解决方案:
12.12 – 7.92
= (12.1 + 7.9) (12.1 – 7.9)
= 20 × 4.2 = 84
问题8.使用(x + a)(x + b)= x 2 +(a + b)x + ab,找到
(i)103×104
解决方案:
103 × 104
= (100 + 3) (100 + 4)
= 1002 + (3 + 4)100 + 12
= 10000 + 700 + 12
= 10712
(ii)5.1×5.2
解决方案:
5.1 × 5.2
= (5 + 0.1) (5 + 0.2)
= 52 + (0.1 + 0.2)5 + 0.1 × 0.2
= 25 + 1.5 + 0.02
= 26.52
(iii)103×98
解决方案:
103 × 98
= (100 + 3) (100 – 2)
= 1002 + (3-2)100 – 6
= 10000 + 100 – 6
= 10094
(iv)9.7×9.8
解决方案:
9.7 × 9.8
= (9 + 0.7) (9 + 0.8)
= 92 + (0.7 + 0.8)9 + 0.56
= 81 + 13.5 + 0.56
= 95.06